It looks to me like you have a well planned itinerary. I am not sure how Frankfurt became part of this discussion.
Regensburg is one of my favorite towns in Germany. Wikipedia and the town website both have good descriptions of what to see and the history (myths included). I love just walking about the old inner core of the city. It’s alleys and plazas are a pleasure to see. The cathedral is massive and interesting. Both it and the nearby stone pedestrian bridge were built in 1140’s. There is a gate to the former Roman fortress “Castra Regina”near both. There is a nice small museum of Regensburg history at the town end of the stone bridge.
I like visiting the church at the former St. Emmeram Abbey. The mythical history regarding St. Emmeram in Wikipedia is worth reading to my taste.
If you enjoy art you might visit the Ostdeutsche Galerie just outside the old town. I like Käthe Kollwitz and her work is featured there.
If you like straight forward Bavarian Food in a friendly atmosphere, try the Regensburg Hofbräuhaus. Kneitinger Bräuerei (sp?) has good food and Bier.
I would suggest that many town websites have valuable tourist and history information. If you have a data plan, the information is nicely portable.