There's a good bit of bus service between Zagreb and Plitvice. I think there's also some from Split, but I haven't traveled that leg. It won't necessarily be easy to get accurate bus schedules before your trip. In the Balkans I've found it safest to go by the bus station as soon as I get to a new town and check out options for moving to my next stop. Often the tourist office can help. (There used to be a T.O. at the bustling Zagreb bus station.) It concerned me that, often when I asked a Balkan tourist office about outbound buses, they picked up the phone and called the bus station rather than turning to their computers to get the information from the internet. I took that as a caution about depending on internet bus schedules for mission-critical situations.
I think it will cost more if you buy your bus ticket from the driver. I was told it would not, but I think I was charged more that the going rate. It is likely you'll be charged a fee (it was around US $1 in 2015) for each piece of luggage stored under the bus. It's not a scam.
Car-rental rates are still higher than they were pre-pandemic in most places discussed on this forum, but if you've been looking at rentals picked up in Austria and dropped off in Croatia, that would always be very pricey because of the international drop charge. It's worth checking on a rental picked up in a place like Zagreb and dropped in Split. That might be affordable.
The trick with Plitvice is that it gets utterly slammed during day-tripping hours to the point that the experience of being there is very seriously degraded. (I've been there three times, as early as the 1980s and as recently as 2015, and the difference was massive.) It's critical to spend a night near the park, which I see you have planned to do. Don't change that!
Dubrovnik is super small in comparison to Vienna (especially) and Prague. Unless you have plans to take it easy in the former and maybe visit some surrounding towns, I'd consider shifting one night away from Durbrovnik to one of the larger cities. (I haven't been to Vienna recently enough or long enough to have an opinion about which needs an extra night more, and in any case that depends on your personal style of sightseeing.)