Wow, this is ambitious even in a pared down form.
For a first Christmas market visit, I would stick to Germany and Austria. Pick cities you think will also be interesting so that when you get burned out of markets, you can go to museums or other attractions, particularly if the weather is bad.
Berlin is a fantastic city for a variety of markets, museums, and things to see and do. I ADORE Berlin. What it does not have is old town charm. So keep that in mind. Larger cities do tend to have more than one market, so you can go to the themed ones. Munich would also be a great choice for variety. I would consider Salzburg and the Wolfgangsee in Austria as well. Nuremberg is famous for a reason, but it can be quite crowded in normal times. Basing in Munich for a few days makes it easy enough to go to both Nuremberg and the local markets without having to change hotels every day, and you could probably do the same for Salzburg / Wolfgangsee if you are ambitious.
We here in Germany are cautiously optimistic about markets, but we expect there may be registration and entry restrictions. That's what they did with the carnival here, and it worked well; there is talk of using the method for Christmas markets. We registered in advance, got "tickets" (free) for a 3 hour window, and had to scan in our tix and vaccine proof. By basing in only two or three cities you can be assured of seeing markets even if you don't get a ticket on your preferred night; it would be a bummer to travel to, say, Rothenburg, only to find tickets gone on your preferred night. In non-Covid times, this would not be an issue.