"...let me know your thoughts."
Our usual style is to rent an apartment to use as a home base in one
or two locations and explore the surrounding areas, but in Germany and
Austria, we think that's not feasible.
This of course would be entirely feasible if you chose to do so; many people do just that. The Dutch and Belgian visitors to Germany often designe their stays in this way; some American "tour" companies make a profit by selling week-long accommodation stays with targeted outings as well. You've chosen differently, for your own reasons, but the fact that Germany is your destination does not dictate shorter stays in more places.
"My husband and I have visited many of these cities before..."
Berlin, Salzburg, Füssen, Nuremberg, Bacharach... these are your planned destinations this time, so which ones are you repeating? And why? There is certainly more to see than what you have seen previously. Maybe you ought to drop the repeats so that all four of you can fly home having explored new places together. There is no "right" or "wrong" set of German destinations, either for one's first trip or one's second trip.
You could quite easily construct an original itinerary with just 2 or 3 bases. Or more if you wish. Do you have any common interests that you'd like to pursue? Just for example...
- Your friends understandably have an interest in their German roots in Bavaria. Pinpoint a town or region of origin and explore the local culture and history there. Here's Bavaria. Which region did their families stem from?
https://www.atlasbig.com/images/bayern-regierungsbezirke-karte.png
- October is harvest season. Do the four of you enjoy wine? If so, maybe the Bavarian wine region (yes, there's more than beer there) of Franken (Franconia) which dominates much of the Main River Valley would be to your liking.
https://i.pinimg.com/originals/10/f5/a5/10f5a59c328e8a5d65f706be784c7e5b.gif
How about biking? Germany's young and old enjoy riding along the country's riverside paths in fall.
History? Which periods? Ancient Rome, WW II, Medieval Times? The Rhine and Mosel regions highlight these periods and others too, but so do other areas.
https://www.cycling-holiday.com/sites/default/files/styles/image_with_text_mobile_scale/public/media/image/file/sw_motm7_2021_0_1_1_1.png.webp?itok=MAtsNgOl
https://www.marksburg.de/en/circuit/#/
- UNESCO has designated 55 different places all around Germany as "World Heritage Sites", places where culture, history, and human achievements can be appreciated. Perhaps a couple of them would be on your route, near your destinations, or special enough to incorporate into your plans:
https://www.germany.travel/en/campaign/world-heritage/home.html
- What about the things Germany is famous for... one or more themes might guide your itinerary.
German automobiles?
Technology and Industry?
Woodworking / violin making / cuckoo clock manufacturing?
Medieval festivals?
The obvious one - Beer? You might need a couple weeks in Franconia...
