I will throw out some random thoughts, ideas and experiences. If anything lights your fire we can go deeper.
Last June we flew into Sophia. Love Sophia, our second visit. Then we went overland to Istanbul. Took about a week but saw some marvelous places. Here are some pictures. https://vasvaripalvacationphotos.shutterfly.com/bulgaria We have a few other places we are obligated to visit, but once we get those out of the way we are off to Bulgaria again. I would only do this with a guide. Rates start about $300 a day for a couple including the hotel rooms. Underground rivers, roman ruins, great ancient towns in surreal surroundings, beautiful people. Very safe and an adventure of a life time.
Romania, spectacular. Again, this is a weeklong trip. This is Eastern Europe before the “westernization”. I’ve only been there once since the change and the progression from the earlier years is still way behind what one would expect. But that’s good. This is a land that will soon change. See it now while you can. https://vasvaripalvacationphotos.shutterfly.com/romania
Slovakia, not on your list but I think one of the great deals in the region right now. In expensive, stunning castles, amazing nature, mountains and ancient mining towns. https://vasvaripalvacationphotos.shutterfly.com/slovakia
Okay after all of the backwater lands you need the comfort of an amazing city https://vasvaripalvacationphotos.shutterfly.com/budapest1
Croatia………………well, in September we make our first trip to Croatia. Runs like this: Land Dubrovnik spend a few days, drive to Perast and spend the night. On to Kotor for a night, Budva for a night and finally Podgorica for a night, catching a flight to Belgrade the next morning. A day in Belgrade and then on to Budapest. I will let you know what I discover. And Ive see a few more in the region but that was a long time ago and probably not relevant today.
Other places you might check out are Hungary, the Czech Republic and Austria. As you can tell I love the region. Another thought would be to set up a base camp in a place like Budapest then use cheap European flights to do your sightseeing. For instance the flight from Belgrade to Budapest was about $75 and we have flown from Budapest to Moscow, Tel Aviv, Paris, London, Sofia, Burgas, etc on low cost carriers and on full service carriers for some pretty low prices.
Anyway……………… if anything looks interesting just ask.