Itinerary help between Munich and Black Forest in September

"Day 5 Black Forest [Sleep Barden-Barden or Black Forest] (Drive 4.5 or 5 hr if last sleep in Garmisch) - Day 6 Baden-Baden [Sleep Baden-Baden] (Drive 1 hr)"

What do you propose to do in Baden-Baden? If you read Rick Steves' suggestions, you know that B-B is about health spa treatments and casinos. There are some high-end hotels, shops and restaurants there too. But if your reasons for going there and sleeping there do not involve spas/casinos, and you're going there only because Rick Steves includes B-B in his chapter on the Black Forest, then I think you need to reconsider. There are dozens of other Black Forest towns that Rick's book overlooks, places with more local charm and character that are almost the exact opposite of flashy B-B. I would look into two nights in the central Black Forest instead.

Gengenbach is one option - an old walled town with cobblestones and towers, good accommodations options and restaurants, and good access to places of interest including the half-timbered town of Schiltach, the Vogtsbauernhof open-air museum, Glass-blowing in Wolfach, Haslach (Black Forest Costume museum.) A day trip to Triberg or Freiburg is also possible from Gengenbach.

"Day 7 Heidelberg Castle, Rhine Valley Bacharach [Sleep Mainz or Bacharach] (Drive Heidelberg 1 hr + Bacharach 1.5 hr + Mainz 1 hr)...Day 8 Would you recommend more Middle Rhine Valley or Frankfurt/ Mainz? (Sleep Mainz)"
Very hectic. (Maybe stop in Mainz for your meal on the way north to the Middle Rhine Valley, or on Day 9, see below.)
"we enjoy more scenery, alphine view, charming town... museum/castle..."
I would skip Heidelberg Castle - OK but not that special. Heidelberg is no small village (150,000 pop.) and it's a very time-consuming detour, especially considering the fine castles you can tour in the Middle Rhine Valley - which has what you are looking for. I'd head straight past H'berg and spend Days and Nights #7 & #8 in one of the towns in the Middle Rhine Valley. Bacharach might be a good choice but there are other nearby options too (St. Goar, Boppard, Oberwesel.)

Marksburg Castle in Braubach – virtually unchanged from Medieval Times

Rheinfels Castle
Landgraf terrace cafe next to Rheinfels
Chairlift in Boppard with a walk to Gedeon's Corner for some refreshments.
Günderodehaus in vineyards near Oberwesel

Then leave Bacharach on the morning of Day 9 (or any Rhine town) for Mainz - spend a few hours in the old town there - then get to FRA for your flight.

Also on Days 5 & 6: you might have time to take a free train ride along the scenic Black Forest Railway to Triberg or another town - one of Germany's most scenic train routes and free with the KONUS guest card you get from your hosts:

Train route near Hornberg
Double-decker trains are good for sightseeing
BF Railway description

Hi Russ, Thanks again for your valuable input and for taking the time to include all the details. This is certainly very helpful. We will likely incorporate a number of your suggestions. Looking forward to the visit!

Thanks Perry for the info! I would definitely look into it. Did you do any trail? If so, how long did it take you?

Lindau.

Thanks Stan for the tip!