On overload beginning to plan . . . need help

Have you been to Venice at all? It's amazing - but also crazy busy with tourists, a different speed than Slovenia and Croatia (which in May won't be too busy yet, except for Dubrovnik). So while you should go...someday...maybe this isn't the time. Perhaps save it for a larger trip to Italy if you've never been.

But Venice Marco Polo airport is a good airport to fly into to start in Slovenia, if you can get good flights in. It can be a busy airport, but it's also not that big. And I think arrivals are easier than departures. I've flown in there twice (never flown out) when that was easier for me than flying directly into Ljubljana (which would have meant an extra connection or a much more expensive ticket or something; last year I got a direct flight from the US to Venice using Alaska miles, something not possible if flying to Ljubljana).

You could just go directly from Marco Polo to Ljubljana, which I've done twice...and skip a stop in Venice entirely. There are services like GoOpti that will get you there. There is also more bus service now (like FlixBus) that has direct buses - some stop at Marco Polo.

Anyway...skipping Venice this time might simplify your planning a bit so you can concentrate on Slovenia and Croatia. Ljubljana is a very low-stress, charming little city, a great place to get over jet lag (v.s. the tourist circus that you have to work to avoid in Venice). I recommend renting a car for at least a day or two in Slovenia. But you can get to popular places like Lake Bled easily by bus from Ljubljana (but you would miss some pretty scenic drives).

I would also rent a car in Croatia for part of the time, especially if you are going to Istria. Take a bus or train from Ljubljana to Istria or elsewhere in Croatia and rent the car there. In 2015, I took a direct train from Ljubljana to Rijeka, Croatia, and rented a car there, then drove about an hour to Rovinj in Istria. (I had a much longer itinerary through Bosnia and Montenegro than you are planning, but you can return the car in Dubrovnik or Split). You want to avoid renting a car in one country and returning it in the other. A one-way rental within Croatia may incur a small one-way drop fee but nothing like renting it in Slovenia and returning in Croatia.

If you haven't already, pick up the Rick Steves Croatia/Slovenia book to learn more about some of these places. You can watch his PBS travel shows for free on YouTube - find the ones on Slovenia and Croatia and see what appeals to you.

We spent 2 weeks in Croatia this past summer with a nice flow in time, this was our itinerary

Saturday arrived Zagreb, pick up rent a car, drive to Rijeka (visit family).
Friday drive to Split - 3 nights, drop off car
Monday catamaran to Korcula - pre purchased tickets
Wednesday ferry and private driver to Dubrovnik (you can take catamaran)
Saturday private driver to Zagreb early morning
Sunday fly home to NY

Notes: you can fly home from Dubrovnik eliminating need for driver or bus ride
Substitute the 6 days in Rijeka for either Zagreb and Pltivice Lakes/Ljubljana
Or a few days in Venice and add time to another city like Dubrovnik and do a few day trips such as Mostar, or Kotor and/or add to split and day trip to Krka Park or Trogir.

We were 6 adults so cost wise the drive was not much more then the ferry or bus

Your trip adding Venice seems too much to me. My husband did a lot of research on the flights using miles.
We were able to use United miles to Ljubljana from SLO, to SF to Munich to Ljubljana. We looked at many options, even Venice. But decided to concentrate on Slovenia and Croatia and were able to snag a good itinerary. Our daughter is meeting us flying from LA and arriving Zagreb on a miles ticket.. Also, we booked our miles with separate one way tickets. We weren't necessarily going to fly in and out of Ljubljana but the best connections worked out that way. Our daughter is flying home out of Split. I think the best idea is to check those one way miles tickets so that you can have options and not have to fly in and out of same city.
If you fly into Ljubljana or even Zagreb you can either head (depending what order you want to do your trip) first to Istria area or to Plitivice Lakes. If from the Lakes is easy to Split where you can ferry to an island or ferry to Dubrovnik. We did Korcula, Hvar and Dubrovnik last time and (while in May maybe not so bad w cruise ships) Dubrovnik was so crowded we are not going this time. I adored Korcula as it seems a smaller quieter Dubrovnik. You could then head up to Istria, which has a Venetian influence, and back for your return flight. Or of course do this in opposite order.
We are going to Vis and Hvar this time and Plitvice for the first time. Istria and Slovenia will be last part of our trip. We do have a car for the entire time for better or worse...It got a little more complicated meeting our daughter there but we do have over 3 weeks!

I also suggest you pick up Rick Steves book on Croatia & Sovenia. We found his info and advice on target and we are there now. We also used it to find places to stay.
We flew miles Seattle, Frankfurt, Dubrovnik. Then will fly back Prague, Frankfurt, Seattle.
Our 3-week itinerary and insights so far - I think May like September is considered "shoulder season" but don't expect low prices and few tourists.
Dubrovnik- We arrived around 18:00 and the hotel proprietor, arranged a taxi and met us at the taxi drop off. After traveling 24hours both were appreciated. We were charged 250 kuna to the Old City (where we stayed) and it was a substantial distance.
Dubrovnik is beautiful and a must see, but it is packed with tourists (us) and very expensive. During the day when the cruise ships are in there are more tourists. We were told by more than one local, tourism is 100%of their industry. We were there 3 nights leaving late afternoon the4th day. Could have cut one day but enjoyed our stay.
Took 2hour ferry to Korcula where we are now. It's heavenly here. We're staying in a simple room with few amenities, an amazing proprietor, and a huge deck with a view of the Sea. The cost is 1/4 of what we paid in Dubrovnik that also had few amenities, but giant amazing space. We can walk out our front door and swim, a walk into town is 5 minutes along the palm lined seaside street.
We have not yet found cheap dining but more reasonable and there are grocery stores and places to buy sandwiches and pizza everywhere. We're here 4 nights, could stay longer; it's relaxing and a fun small community.
Tomorrow we leave for Split by easily accessible, ferry. We expect to join lots of other tourists and it will be expensive. But Diocletian's Palace! 2 nights there, then a bus on to Plitvice Lakes National Park. Yesterday we discovered a "glitch" and had to make a change and reduced our stay to 1 night in a Park hotel with enough time to enter in late afternoon and again early a.m. (To make the most of your time and money get one of 3 Park hotels found on their website.) then we get a bus to Zagreb where we will stay one night in a reasonably priced hotel by the bus station so we can see a bit of the city and have an easy exit by bus to Ljubljana the next day.
We are staying 2 nights in an Air B & B and then will bus to Venice for 2 nights - we've been there before, it's unique, beautiful but also crazy and expensive.
Then we fly to Prague and will be there 4 nights before flying home.
It took me a couple of months of intense planning, research and connecting the dots. It's all worth it - just have to roll with the inevitable travel challenges. Good luck!

If you need to watch your budget, that is reason enough to eliminate Venice.

Thank you, everyone, SO MUCH for your responses . . . just want I've been needing. All help greatly appreciated!

The gorgeous little town of Cavtat is a good alternative to staying in crowded, expensive Dubrovnik -- especially if you are flying into or out of Dubrovnik.

Cavtat is located very near the Dubrovnik airport. Some Cavtat hotels offer free pick-up/delivery, or you can take an inexpensive taxi. It's a beautiful little port with good restaurants. You can take a boat (40? minutes) or a bus (20? minutes) to Dubrovnik to walk the wall and see the sights, and then come back to peaceful Cavtat. One of our favorite places in Croatia.

Villa Andro would be a great place to stay in Cavtat. It's very popular so you must book far in advance ... it's a great, yet inexpensive apartment with a wonderful location.

If you keep Venice, visit Istria. I would not recommend trying to get from north to south in two weeks.
Look at your flight options and then compare the real possibilities.

If I flew into Dubrovnik and out of Ljubljana (or the reverse), would it be reasonable to Dubrovnik/Korcula (4) - Split/Trogir (3) - Istrai, staying in Rovinj (4), Ljubljaja/Bled (3)?

In that case I would skip Istria, to ensure adequate time in Dalmatia (after all, you are likely to return and visit Venice at some point, which pairs well with Istria). Coming from Dubrovnik and heading to Ljubljana, I think Istria is the outlier.

Dubrovnik/Korcula (4) - Split/Trogir (3) - Istrai, staying in Rovinj
(4), Ljubljaja/Bled (3)?

Instead of Istria, I would consider a stop at Plitvice park in addition to adding more time to the other places.

If I flew into Dubrovnik and out of Ljubljana (or the reverse), would it be reasonable to Dubrovnik/Korcula (4) - Split/Trogir (3) - Istrai, staying in Rovinj (4), Ljubljaja/Bled (3)?

I think that would work. I would stop a night at Plitvice on the way from Split to Rovinj. You could do it as a day stop, but even in May you are going to encounter a lot of tourists. You could also steal the night from Split and just leave late - get into Plitvice late afternoon, spend the night, get into the park when it opens first thing in the morning (7am) before the crowds get there, and be on te road for Rovinj by 2-3 PM. Long days in May should help you.

Renting a car in Croatia and dropping it in Slovenia will be expensive, FYI. Either be prepared for a steep one-way drop fee or drop the car in Rovinj and take a bus to Ljubljana - or drive back to Rijeka, drop the car there, and take a train to Ljubljana. I'd recommend having a car in Slovenia too at least for a day. You can rent one just for the day to drive up to Lake Bled and back. The reason to rent a car for Bled is for stops and scenic drives along the way; just getting from Ljubljana to/from Bled is easy by frequent buses.

Any other suggestions from anyone?

This is our first time to Europe (although I'm along in years!) It seems like I maybe should choose between the Dalmatian coastal area or the Istria area.

I guess one of my issues is that I'm drawn to Ljubljana/Lake Bled/Julian Alps, but it's such a long way from Dubrovnik. I know everything is different in its own way, but I'm wondering, at the same time, if I could pick maybe four different areas for 3 (or 4) nights that would sort of represent the best of Croatia and Slovenia. For example, maybe fly into Split and visit Krka and maybe one of the islands (omit Dubrovnik and Plitvice), maybe lunch in Zadar on the way to stay in either Pula or Rovinj - visit smaller coastal cities and maybe inland cities, like Motovun, from there; then go to Slovenia, and either fly home from there or go to Zagreb for 2 nights before coming back to California.

It seems like there are so many options, and I've been reading and watching videos - it seems like no decision would be wrong . . . it all looks so great. Here's something else - my wife loves wine . . . and both my wife and daughter would like to spend a day or two beaching. But, really heavy crowds sort of ruin experiences for us (that's why we're going as early as we can). So maybe someone could speak to those interests/concerns.

And, finally, I'm sure this thread is getting laborious for most everyone, so, I won't keep dragging this out . . . I"ll plan to decide what to do this weekend.

Thanks so much to all of you for your helpful guidance.

I am retired and able to take long trips, but I still face decisions like yours on every trip. In 2017 I didn't have time for Paris. This year I didn't even set foot in Slovakia, which was definitely part of the plan. The more rapidly you zip from place to place, the more crowded your experience is likely to be, because you'll be spending a pretty high percentage of your time at the very same well-known sights everyone else wants to see. If you have a bit of extra time at a destination, it allows you to wander off the beaten path.

I like to limit the geographical extent of my trips enough that I don't blow right past places I really want to see, only to need to return to the very same area in a year or two. So I like the idea of Slovenia, Istria and northern Croatia. Other people prefer to go to their top X destinations, flying between them if they need to. It's really a matter of personal preference.

Yeah the only thing you could do wrong is overdo it and try to pack too much in. Pick the flights that maximize your time, and do what appeals, but skip something (that could be my travel motto).
Mid May will likely be fine for sunning along the coast, but I find the sea a bit chilly at that time of year (of course that's subjective). In late May, it was even cool but more tolerable for swimming. I've been steering clear of the more popular places since my first couple visits, but I don't think you would find the crowds overwhelming until June--with the exception of Dubrovnik, which the powers that be are intent on ruining by not controlling the cruise ships (plus a healthy dose of Game of Thrones mania). I hear Plitvice gets fairly crowded nowadays as well, but I could not dare tell anyone to skip it (I can and do say that about Dubrovnik).

Yes, if we're throwing Plitvice into the mix, we need to say that it is highly, highly recommended that you spend the night before your park visit right in or near the park so you can start your visit at opening time the next day. If you arrive mid- to late-afternoon, you can consider seeing part of the park late that day. It is reported that the hotels inside the park can stamp your entry ticket so you can go back the next day. Otherwise, I believe the park itself sells two-day tickets.

I made the mistake of day-tripping to Plitvice from Zagreb in 2015. It was nightmarishly crowded--wall-to-wall people on the wooden walkways. The walkways are only a foot or two above the water, and it is very shallow, but I still didn't relish the idea of falling in if someone else stumbled, which could easily have happened. I've seen the park under better conditions, and believe me, it's worth whatever itinerary contortions are necessary to see it at its (current) best. I'm sure it will be more crowded than we would like even if you make all the right strategic choices, but you do not want to repeat my 2015 experience.