We did our first Christmas market trip last year. We had been to all the locations previously, so this trip was JUST focused on markets. No other sightseeing. We spent arrival night in Frankfurt and saw all the markets there in about 4 hours.
Next day we took the train to Strasbourg. We spent 4 nights there, traveling by bus & train to Colmar, Ribeauville, Kaysersberg, Obernai & Gengenbach.
Then we travelled by train to Cologne, where we spent 3 nights. The markets there were our favorites. We did a half day trip to Dortmund.
My advice is: pick two cities that have multiple markets and travel by train between them. You are planning much too much moving around. You want to be at the markets when they open (usually 11), or as early as possible, so you can see what is in the stalls and do your shopping. They can get insanely crowded after dark (4pm) and on weekends. Shoulder-to-shoulder crowded. By moving every day you are losing time by having to pack, travel, find parking, drop off your bags, etc.
We are planning a trip in 2025 and considering Stuttgart as a base for 4 nights to see Heidelberg, Tubingen, Esslingen and Ludwigsberg. You might chose Cologne as a second base to see Achen & Monschau.