I was going to suggest some more cities, but they probably are not included in your via rail pass. I've never been, but everyone I know says good things about Budapest. Maybe include it in the last leg of your trip if you have the extra time.

Don't stress about it too much, even the planning. Once you have a rough idea where you want to go, you'll be ok. With internet everywhere, you can book your stuff 2 days in advance and be fine. You'll meet people along the way that are going your route - but in the opposite direction. Ask them for must-visits.

Under-pack. As long as you have your passport, phone and wallet you can buy anything you need on the road in case you forget it. And it's a great excuse to get a couple extra t-shirts or whatever. Bonus - if someone likes it and compliments you on it, you get to say " Ohh I bought this in Bucharest".

I honestly just do a rough research and flag fun places from google/trip advisor/atlas obscura on my maps. Then, every night before bed I do a 30 min research and plan on what I'll do tomorrow. Invite some people along if you got a fun activity planned.

The only thing I'd make sure to look up in advance before leaving are -music festivals/city celebrations (it really sucks coming into a city the day after their annual fun whatever) and some hikes that you might need to make a little detour for.

Have a good trip!