Harried Trip Planning Help!

Time is tight on the itinerary because it has 9 destinations over 19 nights . That’s overscheduling. And you will be traveling 3-4 hours to get to most destinations for mostly 2-night stays. A two-night stay is really a one to one-and-a-half day stay with two nights wrapped around it. Not sure how you are getting from Rovinj to Budapest but public transit will be a time-burner ( 9 hours by bus; 13.5 hours by train) and a car rental will have an expensive one-way drop off charge for picking it up in one country and dropping it off in another. I would suggest dropping Budapest and Rovinj because the order they are in involves backtracking which takes time you really can’t spare. If your return flight is from Vienna, after Postojna Cave you could go to Ljubljana and then take the train to Vienna.

Based on our (couple) experience in Croatia in Aug 2017, one night is plenty for Plitvice Park.; we arrived at 2:30pm and were able to walk the whole park including a climb to a lookout.
I would allocate the extra night to Split or possibly Rovinj. (We stayed in Dubrovnik 2 nights, Split 2 nights, Plitvice Park 1 night, Rovinj 3 nights. Trip Report is posted in Rick Steves Forum in 3 parts: https://community.ricksteves.com/travel-forum/trip-reports/croatia-road-trip-aug-2017-dubrovnik-and-drive-to-split)

Korcula is wonderful, but not easy to get to--or it wasn't when we went years ago. We did Budapest, and then flew to Dubrovnik. Then took a ferry to Korcula and from there, a ferry to Split. Start with the ferry timetable--it used to run just twice a week, meaning you got either 2 days or 4 days on the island. Plitvice is also hard to get to--we drove from Split and it was a longish drive, made longer by road construction. One night, 2 days is plenty, but if driving, you'll need to stay for 2 nights as it is still a ways from anything else. Both Korcula and Plitvice are low on accommodations so start with those--but again, figure out that ferry schedule first (and research all your transportation as that may dictate a certain routing, or even eliminating one of your stops)
Your itinerary is similar to ours (but we did Prague and Krakow and not Vienna) but we did ours in 3 weeks and we were on the go every minute.

Sorry. It’s not an itinerary that flows well, allowing for how long it’s going to take between some destinations.

How are you getting around? A one way car drop will be expensive.

Budapest doesn’t really make sense and your schedule is rushed, so I think you need to investigate other ways to get back to Vienna and drop Budapest.

Alternatively, drop Rovinj which seems to be back tracking. The weather here can be very mixed once you get to the latter part of September.

Ideally, focus on either Croatia or Slovenia.

Thanks everyone for your thoughtful replies. For GeoffB, your trip report was super helpful. Thanks.
A few pieces of transportation info:

1. Taking the ferry from Dubrovnik to Korcula to Split. Have checked ferry timetables. It goes 2-3 times a day, so we are lucky.
2. Picking up the car in Split, driving through itinerary until Rovinj.
3. Driving from Rovinj to Zagreb to drop off car (so the car is dropped off in Croatia, no intercountry fees.)
4. Train from Zagreb to Budapest leaves at 4:30 pm, arrives at 10:30 pm. (There's also a bus but only on Wednesdays.)
5. We actually fly out of Vienna, and it's an easy train trip from Budapest to Vienna. That's why Budapest, although it seems out of the way, actually fits in.

Really appreciate hearing from those who have been there about what cities to spend more/less time. I do have a night that I need to plug in somewhere, thinking either Dubrovnik or Split. Also, according to advice, could take one night away from Plitvice. Thanks all!

I would also suggest dropping Rovinj. It’s too late in the season and Motovun was a big disappointment for us, and my husband is from Croatia! Add the nights and stay in Ljubljana for 2 nights. It’s one hour from Bled and a much better visit. If you’re dropping the car in Zagreb why not stay a night or two there. Very nice city and different from Dalmatia. Also, I would stay 2 nights in Split and one in Korčula. You could even drop Korcula since it is so similar to Dubrovnik. Split is so different you won’t be disappointed.