When I travel in Europe by bus and train, I rarely pre-purchase my train and bus tickets - and I'm pretty frugal. But, the cost savings of early purchase varies greatly in each country. In France and Italy, for example, the fast trains can get very expensive if purchased at the last minute, so if I'm taking one of those I tend to lock at least a few parts of my itinerary in early and buy a few tickets in advance. But in Portugal last year, I didn't buy anything ahead of time, because the trains are not that expensive there, even at the last minute. You aren't going to save 50 euros on a train ticket from Lisbon to Porto, because even at the last minute it costs only about 30 euros. In Italy or France, you might pay 2X or 3X that.
I have been to Poland (not Romania). Almost everything in Poland is cheap. That was the only country where I routinely traveled first class, because (in 2012) the trains were not very nice, and even "first class" was like "second class" on trains in most Western European countries. Maybe the trains have improved since then, though they were still adequate for me then.
I like being flexible in my itinerary though so try to balance between advanced purchase to save money vs. being flexible. And unless I'm saving a lot of money, I won't bother booking anything ahead of time, unless I'm 99% sure I'll be taking those trains or buses.
Flixbus, FYI, uses demand-based pricing (like some train systems), so ticket prices go up as fewer seats remain. But if you happen to be traveling on a bus that isn't very full, the last-minute cost may not be that much more than buying early.