What you want is a transportation hub that will allow you to visit the big city and it's attractions while allowing you to stay in a quieter local. I can think of a few of these, although they are smaller cities.
In Bavaria I suggest Ansbach. Easy to get to and from, with great connections to places like Nuremberg, Munich, Ulm, Ingolstadt, Rothenberg, etc. It was a major location for the US Military until the 1990's (there is still some presence) and it's an attractive place on it's own.
Further west I'll recommend Neustadt (on the Wine Strasse). Easy connections to Heidelberg, Frankfurt, Mainz, Trier, Stuttgart, and even the Alsace-Lorraine. A tourist destination for Europeans in the summer it has the infrastructure and there are dozens of small, picturesque villages within walking distance.
In the Alsace-Lorraine I'll suggest Metz. It's larger than either of the others, but feels small. Beautiful on it's own, easy connections to Paris, Trier, Nancy, Strasbourg, it's an excellent place to spend a week exploring the area.
Take a look at these, and the surrounding areas. They check all the boxes on your list, they can be destinations in their own right, but offer the convenience of access to a lot more of the local area. I've spent weeks in all of them, and in the nearby places, and I could easily do it again.
Disclosure: I've spent the last two Thanksgiving weekends in Paris and Strasbourg, and this year I'll be in Krakow.