If your time is tight (and it sounds like it is) trying to do Plitvice by public transportation won't be efficient. IMHO it's best by rental car.
When we did our Solvenia/Croatia trip, I found the one-way drop-off fee prohibitive (IIRC, like 600 Euro), but that was several years ago and things are always changing, so you need to do your homework on that - without having an actual/current number, it's impossible to compare options.
We were going north-to-south, but we did two cars. We did the crossing along the coast, dropped our Slovenia car in Piran/Portoroz (closest place with a rental car agency I could find), then took a bus a Rovinj. After a couple days in Rovinj, we picked up our Croatia car and moved on. It worked out OK but we did have to burn a few hours with the logistics of the car handoffs and the bus connection. At the time I deemed it worthwhile to avoid the huge fee and give us the freedom of having a car in each country. But it really depends on the specifics of your plans.
We loved the freedom of having a car all through these places. We wandered around Istria a bit (Rovinj, Pula, Motovun), then headed for Plitvice. We arrived there in the afternoon, did a few hours in the park, stayed overnight in the park, got up and did a few more hours in the morning before moving on, and that was perfect. From there we went to Rab (island) which we loved, then back on the mainland headed for Split, stopping at Zadar, Sibenik, Trogir and several other places. There are plenty of interesting places along this coast, which we would have missed without a car.
You don't mention what you're planning between Split and Plitvice, and (perhaps more importantly) between Plitvice and Ljubljana.
We skipped Zagreb but many people give it good reviews. It's also well connected to Ljubljana by public transport. If Zagreb is on your agenda, consider renting a car in Split, heading up towards Plitvice, then after PLitvice head for Zagreb. Drop the car there and take public transit to Ljubljana (or do what we did - do the car handoff/pickup between Rovinj and Piran/Portoroz).
Bottom line: there's no always-right answer. You need to look at your priorities and get actual costs (in money and time) then compare the tradeoffs. Personally, I think having a car (or cars) in this region is ideal (except for the times you are in the middle of old cities, but you just park on the outside and walk in). But it usually boils down to a question of time or money. Which commodity is most constrained for you? For me the answer is easy: I figure I can generally find a little more money, but it's harder to find more days - but everyone's constraints are different. IMHO having a car sometimes can "buy you" extra days (or give you the flexibility to pack in extra places in the same number of days). Again, tastes, experiences, preferences vary.
It's a great area, you'll have fun.