Croatia early October 2019

I would change the balance between Split and Dubrovnik. You can see Dubrovnik in one full day as it’s compact. Trogir near Split is worth half a day.

Once you get into October, many things have closed on the islands so you may prefer to stay in Omis or Makarska on the mainland or take a day trip to Hvar town from Split.

I think your itinerary is generally good. One night at Plitvice is plenty and I also think 2 full days in Split are plenty even if you take a day trip to Trogir. I wouldn't move a day from Dubrovnik to Split as day tours to Mostar run less frequently by mid-October (allow yourself some flexibility) and the addition of a day trip to Montenegro would (imo) be more enjoyable than other day trips from Split. I was in Hvar late September and it was already quiet, which I enjoyed. Where do you plan to return the rental car? If in Dubrovnik, another option (if allowed by your rental car company) would be to drive from Split to Mostar for a night, then to Orebic (short ferry ride to Korcula town) and finally to Dubrovnik. The night in Mostar would replace one of your nights in Dubrovnik. I would suggest stopping for a day in Zagreb, but your itinerary is already plenty full.

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.

The way I read the itinerary is that 5 October is visit Plitvice then drive to Split. Have 6 October in Split and go to Hvar 7 October, which is 1 not 2 days in Split as Frank has indicated.

In re-reading the OP's itinerary I now see that they have 1 full and 1 partial day in Split plus possibly some time the morning of Oct 7 (depends on departure time). Thinking back to my 2 night visit to Split, I still think that's adequate time.

Thanks for the suggestions everyone!