Hello Hikouki, I do not find Frankfurt to be very interesting for historic architecture or a strong feeling of being in Germany. It is mostly modern and focussed on business and banking. I do prefer Mainz for the "feeling"; you can also get good traditional food there. You might consider having one or two of your bags stored at the airport (a bit expensive), or if you can wrestle them onto the train, you will find 3-4 hotels in Mainz wjich are right across the plaza in front of the train station, less than 100 meters away. If you do go there, you might want to find a traditional "Weinstube": Mainz is right in the middle of the wine country.

If you do stay in Frankfurt, check the Restaurant Guru app for recommendation about where to eat; you don't want to end up at a place where only business people or tourists go...