I did a similar itinerary as my first trip to Slovenia and Croatia in October ten years ago - I think I stopped in all of those places (Korcula, not Hvar). I had more time. I wound up driving south from Plitvice to Mostar, dropping the car in Dubrovnik, then taking the bus north to Korcula, catamaran on to Split, and flew out from Split. (Not saying you should do that, obviously you've got your plane tickets.) I didn't do it as you are because I wanted the car for Mostar vs. doing a day trip (Mostar has, I hear, gotten even more touristy since I was there and is busy during the day with tour groups but much quieter at night.)
Not clear on how many nights (vs. days) in which towns, but one night in Plitvice is plenty. Even though I wasn't a huge fan of Zagreb, one night there wouldn't be a mistake - I might take that night from Dubrovnik if you have four(?) nights there - three is plenty really including a day trip to Mostar. It's a lovely town but doesn't take a lot of time to see - mobbed with tour groups during the days (probably even in October - I visited before Game of Thrones made it even more popular), much quieter at night.
So you could take the train say to Zagreb in the afternoon, spend the night in Zagreb, explore in evening before and morning after and rent the car and drive to Plitvice (about two hours) in the afternoon; explore the park the afternoon and next morning and drive on.
Two nights in Split was plenty for me.
You could drop the car in Split and do the rest by catamaran and bus. But you could take a car ferry to Korcula (not to Korcula town, to Vela Luka on the other side of the island) if you want to keep the car. You could conceivably drive from Korcula to Mostar and then drive from there to Dubrovnik and drop the car there, skipping the day trip to Mostar and doing the night there instead.
I'd also rent a car in Ljubljana, something I didn't do on my first visit. Just rent it for a day. You don't need a car to drive to say Lake Bled - easy to get there by bus - but with a car you can enjoy a pretty scenic drive on the way; stop in the town of Skofja Loka and from there, the drive through the mountains via Jamnik and Kropa is scenic and lovely.