Feedback Berlin-Frankfurt drive; Stops & day trips possibilities

For rental car you will need IDP with Texan driving license (regulation). Hope you know German traffic signs and rules.

We enjoy visiting smaller historic towns & cities on a relaxed pace

I would rather spend a few days in Harz mountains or in Saxon Switzerland / Ore mountains with such interest. After Harz you can drive a little detour via Fairy Tale road to Frankfurt area, Be aware that we have two Frankfurts, you need Frankfurt (Main).

Compared to monotone Autobahn you can also drive German Alleys' Route.

Your ideas of day trips are relatively far away - so a lot of backtracking. If you want to stick on Erfurt you do not need a car. Better use trains for this.

In whole Germany you will find a mix of local / German and international kitchen.

Be aware of Sundays and holidays when shops are closed.

Quedlinburg and some close by towns, like Wernigerode and Goslar, are quite picturesque with medieval architecture, some being world heritage, an imperial palace from about 1000AD in Goslar and, as has been recommended, the Harz mountains.

Erfurt, and more generally Thuringia, is also said to be nice, but I lack experience with it. Leipzig is a larger buzzing city, and said to be beautiful, and nearby Halle might also be worth visiting - larger as well, but very different in vibes.

Both options are half way enough that I think it should suffice to have this as your only in-between base for the suggested 3 nights.

If you are looking at larger towns or cities like Erfurt or Leipzig - those should have a fairly well developed international food scenes. Smaller towns will have some (Chinese, Italian, Middle Eastern, Greek or Indian), but probably less sophisticated and varied.

When in May?
These are some events happening in Frankfurt and near-by towns.

Frankfurt Green Sauce Festival 2-9.05
Saalburg Roman Soldiers Encampment 2-3.05
Büdingen Frog Parade (shopping Sunday) 3.05
Teddy Bear Steam Train, Frankfurt Eisener Steg Bridge 10.05
Apfelwein Fest, Hessen Park Open Air Museum 10.05
Worms Medieval Spectaculum Fest 15-17.05
Frankfurt Fressgasse Fest 21-30.05
Frankfurt Wäldches Tag (City Forest) 22-26.05
Pealing of the Bells, Frankfurt 23.05 16:30
Saalburg Roman Soldiers Encampment 24-25.05
Rose Days, Frankfurt Palmengarten 29.05-9.06

I like Erfurt, but Weimar is a culturally richer destination for a multiple night stay, as is Leipzig-but a "city visit.". Quedlinburg is a long drive, and small, but unmissable. Consider the schloss in Gotha, and UNESCO WHS Dessau-Worlitz Gartenreich. I liked Kassel, for contemporary art and a nice UNESCO WHS schloss, with water cascades twice a week, and a fine older art museum on the property.

Watch for strawberry season and chanterelle (pifferling) seasons.

If you have time to skim it, there are some ideas here:
https://community.ricksteves.com/travel-forum/trip-reports/baltic-germany-fachwerke-landesgartenschau

We enjoy visiting smaller historic towns & cities on a relaxed pace bonus if the drives to the places are gorgeous.

Not every driving segment is going to be "gorgeous." Most of the time, you'll be on busy highways that are cluttered with trucks and other cars, and half of you will be busy carrying out the task of driving rather than gazing out the window. I would keep in mind that driving is just a means to get to some gorgeous historic towns.

That said... Below are a few ADAC scenic "Traumstrassen" (dream routes) to consider, which include some of Germany's nicest old towns.

https://www.adac.de/reise-freizeit/reiseplanung/touren-traumstrassen/route-der-rheinromantik/

https://www.adac.de/reise-freizeit/reiseplanung/touren-traumstrassen/deutsche-maerchenstrasse/

Additonal gorgeous old-world towns... This travel blogger has a stunning assortment of half-timbered towns which were mostly undamaged by WW II today appear to some degree as they might have appeared hundreds of years ago. They are scattered around Northern Germany and elsewhere and could keep you busy for weeks, so you'll want to be selective.

https://ninasfachwerkliebe.de/

"Minimal bases"?? I would not be fearful of staying overnight in small German towns. Lodging and dining standards tend to be relatively high all across Germany.

Thank you all for the suggestions & pointers. It's been a lot to read up on. We have reassessed and come up with the following 2 options & would love further feedback.

Option 1:
Day 1: Leave Berlin by train to Erfurt; Overnight in Erfurt
Day 2: Erfurt: Day trip to Weimar and/or Jena by train
Day 3: Pick up Car; Drive towards Heidelberg, stopping either in Gotha/ Eisenach/ Fulda or in Wurzburg (depending on the route we take- anything we should consider here?); Overnight in Heidelberg
Day 4: Heidelberg
Day 5: Train to Frankfurt Airport

Option 2:
Day 1: Leave Berlin & take a train to Bamberg; Overnight in Bamberg
Day 2: Spend more time in Bamberg; Pick up car & Drive towards Heidelberg stopping in Wurzberg; Reach Heidelberg by night & overnight in Heidelberg
Day 3, 4: Heidelberg
Day 5: Train to Frankfurt Airport

We are leaning towards Option 1 since it seems we get to see a bit more of the towns along the way. We are still trying to minimise overnight changes since it is a personal preference & prefer a more relaxed & easy pace. Anything I am missing that I should be aware of/ keep in mind/ think about more?

Thank you again!

I'm a fan of Erfurt.

We enjoy visiting smaller historic towns & cities

Again your mentioned interest: Bamberg has the World Heritage old town. Erfurt is the much larger town and capital of a state.

Option 3 combining 1 & 2:
Day 1: Leave Berlin & take a train to Erfurt; stop-over and overnight in Erfurt
Day 2: Train 45 mins to Bamberg; stop-over and overnight in Bamberg
Day 3: Pick up car & drive towards Heidelberg stopping in Bad Wimpfen; reach Heidelberg by night & overnight in Heidelberg
Day 4: Heidelberg
Day 5: Train to Frankfurt Airport

My suggestions depend on your willingness to deviate to some degree so that these places can be visited and explored. I recommend Meißen, Naumburg an de Saale (especially that of the church), near Potsdam is Werder an der Havel and Babelsberg, should you be interested in film and Prussian-German history, see Schloss Babelsberg.

Potsdam has its Potsdam Museum, ca, few minutes from the train station, after walking across the bridge and a unique museum devoted to the region and city, called , Brandenburg-Preußen Museum.

Re: " absolute radical suggestions " on visiting small towns : First of all, you don't don't have enough time to get to towns connected with WW2 history, stretching from one end of the Greater Berlin area to the other, eg, Neuhardenberg, Wustrau/Brandenburg, Wünstorf, Halbe, Seelow

If your focus is that on history, including WW2 military history, the Greater Berlin and Potsdam area has some singular and poignant places attesting to the horrors of the war, plus other museums and chateaux /Schloesser on Prussian history located.