Acraven always gives good advice. We apparently disagree on two points, doesn’t mean I think I am correct, just my interest. One point is that I ignore borders when convenient. I would never travel to Croatia and skip something that is 10 miles away in a neighboring country just because its not in Croatia. When will I return? To see those falls? Never. So, it’s now or never.
The other place where we don’t share a view is the absolute necessity to spend a night at Plitvice, but you said a few nights didn’t you? If you get an early morning start, if it’s early June, if its mid-week and if you only want to walk and look at wetlands for 3 hours then the daytrip is probably fine. If Plitvice is your dream and it is rightfully for many, then sure, arrive mid-afternoon do a few hours before closing time after the day trippers have gone home, spend the night and start again when the park opens at 7 am ... for another 6 or 7 hours of enjoyment walking. Nothing is wrong with that. You must decide your interests.
Rent a car? Sure, I know people that do it all the time and it works out great for them. We also have maybe one post a month looking for help when it didn’t go so well. The one time I tried it was in South Africa. It didn’t go well for Avis, my car, my S.O. or any of the other cars on the road. Not my first choice now.
The Bus: Yes, that works. But you will drive right past all the things that I enjoy most when I travel. For instance, no bus to the 500 year old village with a really great winery https://mnevillage.gov.me/app/storage/gallery/b1093e9011ffa1322f2e467132d746c8.jpg (also not in Croatia, but i had the photo handy to make the point).
But I avoid buses because they pass what interests me.
Those falls, in another country, are about 10 miles across the border and make a terrific stop to swim and have lunch (there is nice restaurant there). From Dubrovnik to Split I would do Ston, the Kravica Falls (in another country) at a minimum. But the area also has some nice wineries and i believe an olive plantation or two. You may have done wine tasting before, but have you ever done an olive oil tasting. Its fun, educational and something that you will remember. This adds about an hour of driving time compared to driving direct from A to B, and probably less sitting time than the bus. https://maps.app.goo.gl/oYfWtwPDUXDUBdQeA And I forgot, there are oyster farms all up and down the coast from Dubrovnik to Rijeka https://maps.app.goo.gl/JeTkGF2qcgnEhSor9 AND https://maps.app.goo.gl/iKXHZWtVXr8XS3Bf7 One would be a geat stop to eat fresh oysters.
From Split to Plitvice, I would look at stopping at Trogir and Sibenik. Two coastal towns that will give you an idea of what Croatia was like before it became totally engulfed in tourism. Both now are on the tourism radar but nothing like Split or Dubrovnik and still very real.
From Plitvice to Zagreb the first obvious stop is Rastoke especially if you can hit it early in the day. Then lunch in the old town of Karlovac. Heck you could stop and spend a night or two in Rijeka on the way to Zagreb. More wineries in that direction if that interests you.
But I am far from an expert on Croatia. That would take years. For every one place I know there are 100 more I haven’t heard of. That’s why you need to do a lot of research if you like the idea of seeing the little places in between.
EDIT: And do read this for more ideas: https://community.ricksteves.com/travel-forum/croatia/12-day-itinerary-feedback