It will take you a good two hours to drive to Plitvice from Zagreb; longer depending on where in Zagreb you are staying. So you'll want to get a very early start so you can arrive at the park ahead of the massive crowds that can adversely affect your experience there. Alternatively, I'd consider skipping the first night in Zagreb, driving to Plitvice on your arrival day, and spending your first night there. You won't really have enough time to see Zagreb the way you have things planned.
When I was in Croatia, I spent 2 nights each in Hvar and Korcula. I liked them both very much, but you could skip one of them. But I would not add a night in Split. In fact, 3 nights in Split is a lot, even if one of your days there is a day trip to Mostar. I also think an overnight in Mostar is very worthwhile.
It's hard to give you good advice without knowing what you like to see and do. But my itinerary would be one of these:
1 night Zagreb
1 night Plitvice
Next day, wake up, visit the park, then drive to Mostar with late arrival
2 nights Mostar
Next day, drive to Trogir or Split
2 nights Trogir or Split
2 nights Hvar
2 nights Korcula
3 nights Dubrovnik (you can still do your day trip to Montenegro, but three nights in Dubrovnik is plenty)
OPTION 2: same as OPTION 1, but skip Hvar (or maybe do it as a stopover on the way to Korcula). Add 2 nights to Zagreb.
OPTION 3: same as OPTION 2, but add 1 night to Zagreb and 1 night to Korcula.