What time of year?
Even though you don't want/need a car in Ljubljana itself, I'd probably rent the car as you leave Zagreb and keep the car in Slovenia and drive it to Rovinj - that avoids the hassle of trying to rush to catch that bus in Koper. Given that multi-day stays tend to be cheaper than single day car rentals anyway, the extra cost of parking + renting the car for three extra days in Ljubljana might not add up to that much more. One-way car rentals don't incur drop fees in my experience if you return the car in the same country and rent for a minimum of about four days.
Just make sure you buy a vignette driving pass for driving on Slovenia's highways before or very soon after you cross the border into Slovenia!
Plus, if you do that, you won't have that one long mad dash drive between Ljubljana - Bled - Koper and on to Rovinj (wow!). You can instead do a leisurely day trip to Bled from Ljubljana with your car. You don't actually need a car to get to Bled, but I prefer it, so you can do a pretty scenic drive between Skofja Loka and Bled (via Jamnik and Kropa), plus stop in Skofja Loka for a few hours.
I also wouldn't try to do all of Istria in one day - the day you have planned from Rovinj via Pula and Opatija and the hill towns sounds impossible. Again, having and keeping the car means you can split up your day trips over several days.
After Plitvice, I'd probably drive on to Mostar, drop the car in Dubrovnik, then head back north by bus or catamaran to Korcula and then on to Split. That way you won't divide up your time in Split - you end there and can fly out after your time there, instead of visiting earlier than returning to fly out. This is roughly how I did my first trip a few years ago. I did stop for a night between Plitvice and Mostar in the Croatian coastal town of Baska Voda (nice town, not particularly memorable). Doing it again now to split up the drive between Plitvice and Mostar, I'd probably stop in the town of Sibenik (north of Split) which I visited a few years later and loved. If you love churches, I'd stop in Sibenik at least for a few hours on the drive south if you don't spend a night there.
Mostar is a fairly small town. Unless you have specific interests there beyond those of the usual tourist, I'd probably cut that to a single night and use the other night elsewhere. Mostar is a touristy town that gets a lot of day traffic but is more peaceful at night.
I assume you want/need to fly out of Split at the end? If you can fly out of Dubrovnik instead, you could stop in Split on the way south, then Mostar, then Plitvice (ferry Ploce to Trpanjg, then short one Orebić to Korcula town - see a Google map), then drive down to Dubrovnik and drop car, end there and fly out.
Four nights in Dubrovnik is lot for what is a fairly small town (it is unique and lovely), unless you are doing a lot of day trips planned. Dubrovnik gets mobbed during the days with cruise ship tourists - much more peaceful and lovely at night. You can try to predict when cruise ships are in town - there's a website that has a schedule of ships in port. (Heard of it anyway.)
You could do this trip mostly or entirely by bus if you really prefer, but it is a lot more hassle in my opinion. Both Slovenia and Croatia have decent bus systems. I have read of plenty of people doing it and visiting pretty much all of the destinations you plan visit by bus and ferry. Still, I've found driving in both Slovenia and Croatia to be easy and affordable.