I mean, if you're just looking to see the Christmas markets, you could do each of those places in one or two nights.
The markets in Salzburg are fantastic. I went last year and expected to see more or less the same faire as you'd see in the rest of that area, but they were truly unique. Lots of unique items made with local resources (and excellent music all around). If you're in Salzburg, there are quite a few smaller places with nice little markets in the vicinity, such as Sankt Leonhard. I went last year, and did a day trip to the markets in Hallstatt as well, which were lovely (albeit tiny).
Strasbourg goes all out at Christmas. The markets are really good (not spectacular, though), but the decor in the old town is gorgeous. The city bills itself as the Capital of Christmas. If you're staying multiple nights, I'd do a day trip over to Colmar. I find it even more beautiful than Strasbourg, and it has a good market scene as well.
I'm not a fan of the market in Nuremberg. I went in 2019, and it was just one big market with stall after stall of pretty much the same thing. I was pretty disappointed. The Frankfort market scene is pretty small and underwhelming as well.
I'm adding Munich and Innsbruck to my market trip this year as well, so I'm super excited. I've been to 40+ markets the past 4-5 years, and my top 3 cities for markets are Vienna (by far), Prague, and Cologne.
Have fun!