I'm going to preface this with the knowledge that due to COVID, I know the Christmas markets may not open and the trip may not happen.
We are supposed to fly into Frankfurt on Dec 11 and out of Luxembourg City on Dec 22. It was a whim airfare purchase (cheap, good flight times) and we we've wanted to visit the Christmas markets for awhile. After reading about them, we're now trying to base the trips on cities that we want to see foremost that also have good Christmas markets (especially indoor activities if it's cold/cruddy weather). I'd like to do one big city and we're trying to decide between Berlin or Vienna. Berlin is obviously closer to Frankfurt, but is it really nice at Christmas? We haven't been to either but Vienna just seems more Christmas-y to me. We'd also like a smaller city, thinking Stuttgart (we'd love to see the car museums there) and a smaller town that is just for a good market, maybe Trier before a few nights in Lux.
We have 10 nights and would like to reserve 2 for Lux - just so we can see a bit of the town before leaving on a morning flight on the 22nd. Where would you spend the other 8 nights? We prefer train travel (we've driven in Germany and just don't feel like doing that in winter) and don't really want one base to daytrip from. We've already seen Munich and the Freiburg area.