Itinerary in Slovenia and Croatia

A day less in Dubrovnik and a day more in Ljubljana. Or, while in Dubrovnik consider a day trip to Kotor.

My first impression is that you have a lot of doubling back, because you are driving all the way south to Dubrovnik and then back north. So I would ask if you are locked into your flights, because it would make much more sense if you can fly open jaw. The only issue you would then need to deal with is to return your car in the same country where you rent it in order to avoid a high surcharge.

Have you considered flying into Dubrovnik and making your way north, then flying out of Zagreb? (Or the reverse of that?)

Also, you are sleeping 4 nights in Dubrovnik? What are you planning to do? Unless you have several day trips planned, that's about 2 days too long. You might consider subtracting a night from Dubrovnik and adding it to Mostar.

If you get settled into your lodgings near Plitvice on May 6, you might be able to see part of the park on May 6 during the late afternoon, after most of the day-trippers have decamped. Don't try to do it all that day by starting earlier, because you will definitely not see the park at its best. Plan to get an early start on May 7 and beat the crowds onto the walkways. It will make a huge difference in your enjoyment of the park, which is now extremely popular to the point that it is not a good experience during periods of peak visitation.

You have a lot of time allotted to Dubrovnik. I assume you are planning at least one if not two side-trips? Mostar and Kotor are two possibilities.

I like Zadar a lot, but I think some hours on May 14 and some hours on May 15 will be sufficient, allowing you to move on to Istria on May 15. In Zadar, late on May 14 is the time to see the Greeting to the Sun along the waterfront. It's an after-sundown experience.

Istria really could use some more time. In addition to Rovinj you have the (very different, though equally touristy) town of Porec, a cute little coast spot called Vrsar, and two historic inland towns (Groznjan and Motovun). There are some Roman ruins in Pula, though no one on the forum has seemed very excited by them. Inland Istria is very pretty and a different experience from the coast.

It doesn't appear to me that you have much time in Ljubljana at all. It's a very nice city. I think there might be some sort of art/craft market along the river on Sunday, May 20, but I'm not sure.

You could use more time for rural Slovenia, yes.

Don't forget to clear with your rental-car company your plans to drive into Croatia and Bosnis-Hercegovina. And Montenegro if you opt to see the fabulous Bay of Kotor.

You should anticipate parking challenges at many of your destinations, so you might do an initial lodging screen on a site like booking.com where you can set a "Parking" filter.

Edited to add: Lane's point about the route is a very good one. Zagreb and Ljubljana are easily linked by train or bus, and it should be at least as easy to fly into or out of Zagreb as Ljubljana.

On your way from Rovinj to Bled you should consider visiting Istrian hill towns Groznjan and Motovun and can easily do both in a halfday.

a lot of doubling back, I agree. But if you booked flight already is fine.

If you used one direction u could see more of the south beauty of islands.

As a travel consultant for 25+ years (and traveler to this region), my suggestion echos a few above with a little tweak.
Travel ONE WAY - start in Slovenia. See Ljubljana ... then rent your car and travel to other Slovenian destinations.
Turn in the car in Slovenia.
It costs so much more for an international one way drop off. But one way drop offs IN country generally have no penalty. (Auto Europe gives great prices for Budget, Avis, Hertz, Europcar, etc. - no ... lol I do NOT work for them)
You could easily rent the car in Ljubljana and return it there, too.
Because you can then take a train or bus over the border to Croatia and rent a different car in that country.
I would recommend Croatian pickup in Rovinj. (After you see that city - you don't need one IN the city)
and you could drive south to Dubrovnik and turn in the car there when you first arrive (you won't need one there).

IF your flights are booked already round-trip to Ljubljana
- no worries you can get a non-stop flight from Dubrovnik back to Ljubljana
Adria Airlines has flights Dubrovnik to Ljubljana as low as $128 one way.
This would be cheaper than bus, train combos.
Adria Airlines website in English: https://www.adria.si/en/?SaveCountry=1

This is a smooth way to travel.