18 days in Germany Frankfurt to Munich

You have some backtracking in your plan at the moment. Rothenberg is very near Nuremberg/Bamberg/Wurzburg. I would flip-flop your Heidelberg/Bade-Baden leg with your Rothenberg leg.

While Freiburg is amazing, it is a bit far south and out of the way. I would stay closer to Baden-Baden. If you like pottery, there is a charming town right across the board from Baden-Baden called Soufflenheim that sells Alsace pottery.

If you are going to Baden-Baden for the spa experience, maybe skip Baden-Baden and go to Bad Kissingen (a bit north of Wurzburg) They have a fantastic spa there, that we highly enjoyed. KissSalis Therme Bad Kissingen

We really liked Bayreuth about 45 mins from Bamberg, there was a palace called the Ermitage and the Opera house there was amazing.

In order to eliminate as much back tracking as possible, try to arrange your stops in as straight a line as can be done despite the zigs and zags. Consider going from Oberwesel to Wurzburg. Then Heidelberg, Rothenburg, and Nurnberg. Just a thought.

There are numerous spa towns in Germany near Rothenburg and Nuremberg as well as handsome old-world towns and castles. There's no strong reason I can see for you to be doing the large detour to Heidelberg and Baden-Baden. The links below will provide some info on the spa options and other attractions in these places. All have train stations.

Bad Kissingen
Bad Windsheim (don't miss the Franconian Freilandmuseum)
Bad Staffelstein
Bad Mergentheim

Additional historic towns near Nuremberg that your itinerary is missing and that rival or surpass Heidelberg:

Bamberg, Regensburg, and Bayreuth (all 3 have UNESCO World Heritage credentials and are day trips from Nuremberg by train. Coburg, home of Veste Coburg, is an option as well.)

For scenic towns with history, consider the following:
Coburg: The Coburg nobility managed to marry into most of the ruling families of Europe - most notably Prince Albert married Queen Victoria. The town includes the Veste Coburg (one of the best medieval fortresses in Germany if not the best), Ehrenberg Palace, a beautiful town square (with great bratwurst and bakeries), and other points of historic or scenic interest. I've never encountered more than a handful of tourists in the summer. Coburg was tucked in a dead-end corner of West Germany close to the old East German border so it avoided a lot of tourist traffic and its reputation still seems muted.
Sesslach: A restored, walled medieval village about 20 minutes from Coburg. A couple of good to great traditional restaurants. A great bakery. Very scenic - a small Rothenberg without many tourists.
Ummerstadt: If you love very old half-timbered buildings, nothing modern, and no gift shops, this tucked-away village just outside Coburg will satisfy your interest. You can see it quickly and you won't see a tourist unless you look in the mirror. I believe there is one restaurant in town - very traditional and good food.

North of N’berg Kulmbach is another town with a nice castle. Kronach looks like another in the area but I didn’t make it to there on my trip.