thoughts on itineary

Okay, first, what is the tour and how long does it last? I'm a little confused by your schedule. See if this is accurate:

9/23: 2 nights in Munich
9/25: Drive to Regensburg, sleep in Regensburg 2 nights?
9/27: Depart on tour 2 nights? What kind of tour? More details would be helpful
9/29: Drive to Würzburg. Sleep where? Are you sleeping in Würzburg or Augsburg?
10/1: Drive to Fussen. Sleep in Fussen for 3 nights?
10/4: Open 2 nights
10/6: Drive to Munich, return car, and sleep
10/6: Fly home from Munich.

Okay, my first question is why are you driving? Most of these places are much better reached by train. It will be easier and cheaper. A the very least, I would omit the car for part of this.

For example, take the train from Munich to Regensburg and stay there while you visit Regensburg and Würzburg. Then maybe pick up a car to drive to Fussen. But more info would be helpful as your itinerary is a little confusing right now. :-)

Okay, here is one more thought. I would consider spending your free nights in Salzburg, which is a beautiful location and has so much to see and do there. It's one of my favorite cities in that region. In fact, I would pull a night from Fussen and add it to Salzburg, but that's me. It really is a wonderful place to visit.

Drive to Hinterhornbach, Österreich from Füssen and walk to the Petersbergalm. Should be open till 12 October. Stunning valley. @12k walk there and return from the parking place. Double check that they are open and opening times.
https://www.petersbergalm.at/

Mardee, I think you're misreading OP's use of the word "tour." I think the itinerary is:
9/23: 2 nights in Munich
9/25: Drive to Regensburg, activity is "tour area," sleep in Regensburg 2 nights
9/27: Drive to Würzburg. Again, "tour area" is the activity. Sleep Wurzburg
9/29: Drive to Augsburg, "tour area" and Sleep Augsburg
10/1: Drive to Fussen. Sleep in Fussen for 3 nights
10/4: Open 2 nights
10/6: Drive to Munich, return car, and sleep
10/7: Fly home from Munich.
I think OP needs the car to tour the areas. Lodging is booked, so the question seems to be how to fill the two nights between Fussen and return to Munich. Salzburg is a great suggestion if OP can get rid of the car, which seems impractical.
How about Innsbruck? Or a smaller city/town nearby?

Oh, got it. I thank you for the clarification!

I didn’t get much sleep last night. Maybe I need a nap. :-)

For the missing day, drive via Innsbruck, Krimml Falls, and Werfen to Salzburg. In Werfen perhaps visit the ice cave

From Augsburg go to Bodensee. From there you can tour Fussen (or ignore it), and you can easily spend 4-5 days around the lake and not see everything. that takes care of the unplanned days also.