Rick, I believe you had a journey like this listed at one time.
Do you still offer such group tours?
I'd also like groups that are at a little slower pace for those of us a generation older than you
What is available?
The best way to get answers to your questions would be to contact the tour department directly by phone or email:
https://www.ricksteves.com/about-us/contact-us
jb,
If you'd like to peruse Rick's tours, have a look at the tours section of this website....
https://www.ricksteves.com/tours
You might also have a look at the tour experience videos to get some idea whether the pace of the Rick Steves tours would fit your preferences.
There appear to be several tours available on the theme that you mentioned. This is one of them....
http://www.luthertours.com/travel/item/94-in-the-footsteps-of-martin-luther-10-days.html