Those cities you listed are definitely worth going to (maybe with the exception of Luxembourg?), but they're rather touristy and maybe not what you're going to think of as "local"... They're the first stops for your pedestrian NA tourist.

I think that's a logical list for now, maybe stopping in some small towns when taking train trips between cities. I don't recommend spreading yourself too thin, as it's better to spend a few days in one place and find those hidden gems at your own pace. I would add Munich, Budapest and Belgium (not Brussels, but Ghent/Antwerp/Bruges)

I spent a good amount of time backpacking solo in the Baltics and Balkans this past summer and it was fantastic. You meet so many awesome people along the way and I love the freedom of doing whatever I feel like everyday.

Hostels are 100% safe as long as you exercise common sense precautions. Don't flash your valuables and keep them hidden or locked up. Go for hostels with at least 90%+ ratings and more than a few dozen ratings on the major sites and it's hard to go wrong. Make sure you research the location too! It's a great experience but not the most "plush" for sure. All I care about is having a place to crash for the night, so it fits the bill for me.