With two full days (Friday and Saturday) to see "all of" Dubrovnik, you could do it. It depends, though, on what is "all of" is for you. If you want to walk the wall, stroll the Stradun and take the cable car up to Mt. Srd, a day is enough. If you want to delve slightly deeper and visit the Ethnographic Museum, the cathedral, the two museums/exhibits about Dubrovnik during the war (one on the Stradun, one on Mt Srd), the Serbian Orthodox church, the maritime museum, or other "lesser" sites, you'll need two days. Also, if you're a Game of Thrones fan, you'll probably want time to spend in the exhibit and/or tours. Based on your choices there, you could then make a Kotor decision. There's currently another thread on Kotor day trips vs staying over there. There were no visa requirements when we visited, but it is always a good idea to check the State Department website on such matters. Of course, you'll need to keep current on the COVID entry requirements as well.
I think that Split in 3 days is just about right. Diocletian's Palace (and the attached Cathedral of St. Domnius) is the main draw, but the Ethnographic Museum in Split is also worth the time. If you're not going to Zagreb, I'd make time to see the Mestrovic Gallery. Mestrovic was Croatia's most important artist and his work can be seen all over the country as well as in the US.
From where would you day trip to Plitvice? From Split it's 3 hours. I suppose the best way to do it would be on a bus tour, but if you've got the time, the park deserves a day. If you can manage an overnight somehow, that would allow you to arrive at the park in the early morning, thus beating the masses of tourists that arrive mid-day.. Plitvice requires advance-purchase tickets, so plan ahead.
