The average temperature for May 30 in Dubrovnik (your mid-point) is 52F to 69F. That’s average so I would plan on temps 5F below and 5F above. So 47F to 74F. Today’s projected is 55F to 65F. Ljubljana was a bit cooler yesterday 41F to 66F. In Dubrovnik in April and May it’s going to be overcast close to half the time you are there, vs 30% decreasing below 20% as June progresses. Rain chance is 20% to 30% dropping to near 10% in June. So, a coat and a rain cover make sense for the trip. And those boats to the islands, pick one that has an indoor cabin. An extra layer for the morning hikes/walks is probably sensible.
Plan a lot of indoor activities too, that will help, especially in the mornings. I am lucky there, I like wineries and there are a lot in Montenegro and Croatia. All of the cities on your list have very nice hotels if the weather changes that could be a nice excuse to hold up in the hotel and rest.
Durmitor National Park is very pretty and there is just one more layer of clothing to enjoy. Easily done. Averages in April are 35F to 55F but close to 70F by June.
I did go to Montenegro last May. https://community.ricksteves.com/travel-forum/montenegro/a-week-trup-report I live close, so the cost of the risk is low, and my plans were trout fishing in the high country. I got rained out the first day and went to the coast where it’s a bit dryer. But it was too early in the year for my interests to enjoy much, the weather was miserable for my taste (pretty much the averages) and other things came up so I took an earlier flight home; 40€. I used the same travel company I always use for Montenegro. I just called with the good news; they got me out of my hotel obligations and sent me a transfer to the airport. Not much trouble. This year I put it off until 11 June.
Still among my favorite places in Europe and if you go in with the right attitude and expectations you will enjoy; and I cant think of a better time to beat the tourist crunch. You have it well worked out, go have a blast.