I think you will want to add a night at Plitvice instead of doing a day stop on the way to Split. If you visit mid-day, you'll visit at peak tourist group time and have to compete with a bunch of tour groups - not very pleasant, kind of feels like Disneyland. If you add the night, you can stay in the park, take your time, and see some of the park the first day, some the next morning and have a much better experience.
That gives you about four nights at the end after Dubrovnik to get back to Venice. That's reasonable though that means a lot of long days of driving - seeing what? Its kind of wasted time unless you have time to stop and enjoy places.
As I suggested, you could instead ditch the car in Dubrovnik and fly to Zagreb, maybe spend a night there, take a train to Ljubljana (really lovely city) and spend a few nights there, then fly out of Venice (take a GoOpti shuttle or bus from Ljubljana to Venice, depending on the departure time of your Venice flight out. I had a noon back to the US out of Venice last October and the GoOpti shuttle that morning worked fine, about a three hour ride).
And on the way in, take the bus or a GoOpti shuttle from Venice to Rovinj. That avoids having to drive a few hours right after a long overnight flight where you might be tired. You probably won't be able to rent with National, but the cost to rent in Croatia may be cheaper. And most rental cars rented in Croatia make an exception for driving through the Neum corridor (Bosnia) on the way from Split to Dubrovnik. Maybe National does too, but I would absolutely confirm this very clearly ahead of time.