12 days Croatia with car Zagreb Split Hvar Dubrovik
 

Planning a trip for Oct -
2 nights Zagreb, then pickup car
1 night at Plitvicka
3 nights Split -day trips to Trogir, Sibenik
2 nights Hvar -via car ferry
3 nights Dubrovnic via Hvar Island to mainland ferry, then drive along coast.

It seems that having a car in Split, Hvar and Dubrovnik is not really convenient however we like the flexibility of having a vehicle. Would like to visit Korcula but maybe because of season (Oct) and logistics (car from Hvar) this should be left out.
Trying to determine best routing (driving) to see some of the islands and scenery without feeling too rushed. We could possibly stay extra night/s in Split and daytrip to Hvar and Vis or Brac. Or daytrip to Korcula from Hvar by catamaran.
Or maybe there are some towns/sights that we should see instead.
Any ideas/suggestions?

You couldn’t pay me to drive rather than take the (comfortable, convenient) bus from Split to Dubronik – that stretch one of the most stunningly scenic routes in the world, IMO, and I am very glad that I was able to take in every bit of it -- which I couldn’t have done that if driving. And that’s particularly true because you will end up in Dubrovnik, where a car will likely serve no particular purpose. But that’s just me.

And I don't see a lot of spare time in your plan for driving around, rather than just driving directly from place to place -- but maybe I'm not understanding what you want to do in the places you do plan to visit. If you are sure you have the time to roam, try viamichelin.com or googlemaps to explore your options.

3 nights is a lot for Dubrovnik, particularly in comparison to the rest of your trip's pace – you might want to shift another night to Split (there’s a lot to do there, and a lot of day trip options) or Hvar (where you really have only 1 day, if I’m following correctly).

Good luck!