Just a few thoughts:
Be sure your rental car is OK to go into Bosnia and Montenegro. It may well just be a matter of clearing your plan at the time you make the rental reservation, but best to be sure.
While you're in Rovinj I hope you'll have time to see Porec (quick stop in Vrsar on the way), Pula if you're interested in Roman ruins, and some of the historic towns in the interior. The latter are difficult without a car.
Four days in Dubrovnik feels sort of long to me, even with one day for Kotor and Perast. Check on cruise ships expected in Dubrovnik and Kotor during your stay and try to schedule yourself to avoid the worst days. Kotor is very picturesque (far better when there's not a monster ship in port), but it's not the only worthwhile town in Montenegro, which is a lovely little country. I enjoyed my day-trips to Herceg-Novi, Ulcinj (Turkish vibe), Cetinje, and Budva, though the latter is a bit like Kotor and wouldn't be my top suggestion for an additional stop. None of those places is more than a 90-minute drive from Kotor. If you chose to spend a night or two or three in Montenegro, you could see a lot with a car (which I didn't have). I've heard that the Lovcen and Durmitor National Parks are lovely.
Back in Croatia, here are a couple of places you didn't mention that are worth some time if you end up with excess days:
Zadar, on the coast north of Split: Very picturesque old quarter on a peninsula. Quite touristy, but for a reason.
Zagreb: It has a large multi-level historic district that feels very different from the coastal cities. Active café culture. Multiple interesting museums. Very much worth a visit. I'd choose time in Zagreb over Zadar because Zagreb is more different from your other destinations.
Sorry, can't help on Brac. Have only been to Hvar and Rab, and too long ago to have useful info.
