First time itinerary

First check the ferry schedule carefully because I think it is seasonal and not daily, but that is how I would approach it.
You could rent the car in Pula. Stay wherever you like in Istria (a few days, if interested in hill tows and such), then drive to Plitvice area for a night (most say it is best to spend late afternoon and early morning in the park), then head to the Krka area where you could stay in Sibenik or somewhere rural. You might read up on the Kornati islands and see if that is something you might want to do as well. So, depending on your personal list, a few days in this area.
You might want to at least see Diocletian's palace in Split because it is really cool.
Last, end with a few days in the Korcula and Dubrovnik areas.
Plitvice and Krka are very famous and crowded, but there are many other parks in the country, so I would encourage you to look into those.

You aren't interested in big cities but are you okay with cities where there is double digit tourist numbers for every local? Google Croatia over tourism. Sounds like costs are exploding, European tourists are avoiding it and US touristd are still flocking in.

Im not saying not to go. I was there, again, in September. I had a great time. But I knew what to expect and I live relatively close. I still love Dubrovnik for some reason. But for every day I spent in Croatia this year I spent 5 in Montenegro / Bosnia & Herzegovina to escape the tourists and enjoy the nature and villages with few tourists.

I don't know about the ferry logistics, but this sounds like a great trip. Rent a car in Pula or Rovinj to explore Istria for a few days, skip Zagreb and Split if they don't interest you. I recommend spending the night before Plitvice in Rastoke. It's a beautiful area to walk around for a few hours. Get to Plitvice as early as you can. We spent 3 nights in an apartment in Skradin and loved it there. We spent a day at Krka NP (not as impressive as Plitvice IMO) and another day visited Sibenik for a few hours and the nearby Falconry Center, which was fantastic. We also spent a night in Lovran, which is a very cute small town along the water. We spent 3 nights in Korcula. It was fun but I suggest considering 2 or 3 nights in Mostar instead. Or pop into Slovenia for a few days. Drop off the car in Dubrovnik and spend a few nights there.

Thanks for your replies!

@MR Ê 🇺🇸 🇺🇦 🇭🇺, I hear you. While my preference is smaller towns (this is partially why we didn't consider Split and Zagreb), sometimes you just have to suck it up in the crowds to see the major sites. I don't want to move around too frequently and not being familiar with those countries at all, we will definitely consider branching out more in the next few years.

@cindyeb and @ valadelphia, I appreciate your specific suggestions. It looks like the ferry service runs through September.
Why Mostar over Korcula?
I like the variety of looking into Kornati Islands over Krka although rather than a few hours, it will require a few more nights' stay. Any recommendations on lodging on those islands? And we do love the parks so any additional first-hand advice you have would be appreciated!

Sounds like you understand things and have focus i suspect you will have a great trip. Korcula was very okay for me, more so than Mostar. Just never found the value in Mostar ... but I love other stops in Bosnia & Herzegoivna. You might get tempted to go to Kotor, I would pass on that too and work Croatia really hard.

I can recommend Zrinka House for your stay near Plitvice. It is a beautiful B&B that is impeccably run by Zrinka and her daughter Barbara. You can find it on booking.com. They even packed us a lunch on the morning that we left. Highly recommend! We absolutely loved our time on Korcula. You can spend a day taking the hop on hop off water taxi to some of the smaller islands. If you’re into wine, there are great wineries to visit. We were there on an evening when the Moreska Sword Dance was performed, and it was quite entertaining. We had a two bedroom, modern apartment with a gorgeous view of the harbor and old town. Magical! Split was our least favorite location in Croatia, so IMO you won’t regret omitting that from your itinerary.

I think you need to pinpoint what you plan to do/see at stops to decide where to go. We spent 3 nights on Korcula and it was nice but not really different from other towns in Croatia. We spent a fun day on the Hop On/Hop Off boat. We spent 2 nights in Mostar and it was quite different from Croatia. We also prefer small towns and Skradin was small and beautiful. We could have skipped Split and Zagreb. We did not understand the appeal of Trogir but Primosten was a nice stop and looked like it would be nice to stay at if you like beaches. Another thing to consider is drive Istria--Plitvice---Krka---Split. Drop off the car in Split and ferry to Dubrovnik, stopping at island(s) that interest you along the way. There are so many great options.

We went in June 2024, (going back this August), started in Venice for a few nights, then took train/bus down to Rovinj for three nighs. I looked at ferry from Venice but the only times then were late day and didn't want to arrive in Rovinj that late. Picked up rental car in Rovinj and drove to Plitvice for one night. Got a two day pass for Plitvice so we could leisurely see everything in a late afternoon/morning. Drove to Split for one night where dropped off the car at Sixtx location not far from downtown. Found the driving to/from Plitvice very easy. Then went to Korcula for 3 nights, then Dubrovnik for 3 nights where we flew home from. I would say you don't need a car in Korcula or Dubrovnik. Overall our whole family loved the trip and we've been to many other places in Europe. Great combination of sights and leisure. We are going back to spend more time adding a couple islands this year and are adding in Montenegro as well.

@kpapagelis, I agree that arriving in Rovinj at 8pm is not ideal, however, I've tried a number of searches and cannot see any train schedules, just the Flixbus that takes 7 hours. Can you help?
After other considerations, we are now considering a more extended itinerary so I created a second post if you have any other thoughts: https://community.ricksteves.com/travel-forum/croatia/2nd-pass-at-itinerary

Thanks!

Hi, sorry should have been clearer, we took train from Venice to Trieste (~2 hrs) at around 10:00. Then Flix bus from Trieste to Rovinj which was also ~2 hrs. The bus station and train station in Trieste are right next to one another. I think we waited an hour or two for the bus, but just walked around Trieste a bit to kill some time. This worked well for us, but I do see how the ferry would be more relaxing and simpler, just need to work out with your accommodation around arrival time if getting in late.