Lots of good thoughts here. I actually really liked Plitvice. But my experience was in 2021 and I have only been once - so that doesn’t make me expert. When I went, tourism was still slower after Covid. The park opened at 8 and I was in at 8:30, did circuit C slowly, and was back in the car heading to Split at 3. I did some reading last night and apparently it now opens at 7 (you’d want to double check that). I have seen pictures of what acraven describes, though. If it is on your absolutely-want-to-see list, then you spend a night there before you go and be in as soon as it opens. That would be your extra night.
I also liked Split and spent 3 nights, I am not sure about the GoT sights - I probably mistakenly thought there were more in Dubrovnik. It’s conceivable you could get by with 2 nights in Split.
If you drive Split to Dubrovnik, you will have to pay rental costs for days when you park and don’t drive, PLUS find lodging with parking.
If you return the car in Split and get another in Dubrovnik, you can take the ferry Split to Dubrovnik AND stay right in the old city of both. Or from Dubrovnik, opt for a driver. You might have to do some number crunching.
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Montenegro is gorgeous. I spent a slow 2 weeks there with a rental car. The biggest disadvantage was that I found few pull-out places to admire the view. Roads are fine but narrow - by the coast it’s crowded and many places away from the coast are mountainous. It doesn’t allow for a lot of looking around for the driver (me). So a driver isn’t a ridiculous idea. I had 3 days in that 2 weeks with a guide/driver (plus a transfer Dubrovnik to the BnB near Podgorica) and they were all 100% worth it.
Border crossings are not scary at all - you just have to have the required papers for your rental car, which the rental company should have ready for you. However, they CAN be time-consuming. We took the longer route but shorter crossing time via a northern crossing (wish I could tell you the name but I think through/by Trebinje) but the road took us past this amazing spot https://maps.app.goo.gl/2pxfM1LiRRtHKqAa8?g_st=com.google.maps.preview.copy. If you go this route, you will go past, and should have time for Ostrog Monastery. (And MAYBE past Mr. É’s favorite restaurant there: Sokoline Mali Hotel Restaurant. That would be a long day but doable.)
Then spend the night near Skadar Lake and spend the next day there. If you finish with time to spare, the beach at Petrovac is not far. It’s not one of Montenegro’s finest but it’s nice if you only have a few hours.
Then fly from Podgorica. If you fly from Dubrovnik, it will require a night in Dubrovnik before your flight, but you could spend that day driving back along the gorgeous Bay of Kotor. That will be a full day, with a couple of stops - you just have to decide where. Ha!
If you have an extra night to allocate to Montenegro as you are whizzing through, then take acraven’s advice and spend it in Durmitor National Park on the day you leave Dubrovnik. I stayed in an Etno village (cabin) above Piva River Canyon and Mr. É has a great hotel recommendation for the Tara River Canyon - just opposite sides of the park. You should have a half day to drive some of the loop or hike. (Mr. É’s recommendation, which I don’t remember, might be better.)
Just remember, it’s not possible to see everything that deserves to be seen. Edit: I was writing at the same time as Mr. É. He has a point worth considering.