Thoughts on this itinerary 9 days in September 2020

My advice is to take the train to Paris from Frankfurt instead of flying. Faster and cheaper. One way tickets are usually 39€ and the train takes a bit less than 4 hours. You will spend that much time getting to the airport 1.5 hours early, the flight, deplaning, and getting into Paris from the airport, plus the costs for all of that transport.

Day 4 looks really full. Not sure if this is enough time to explore 3 towns, plus driving between them?

I would stay in Tubingen rather than Reutlingen

I agree that you should take the train to Paris instead of flying. But it's not necessary to do it from Frankfurt, there are several other cities in the area with direct trains to Paris, like Köln, Mannhein, Kaiserslautern or Saarbrücken. And the drive from Zell so Saarbrücken is not that much longer (10-15 minutes or so) than the drive to Frankfurt, and from Saarbrücken it's only 1:53 to Paris by TGV.

Most restaurant waiters in Germany answer me in English. Don’t give your German another thought.

We loved Traben-Trarbach, (very popular with bicyclists) and it is less touristy than B-K. But you have to reckon with the fact that these are not undiscovered villages that the American GI’s just cleared out of. I would not try to do more that two towns a day. I’ve done 3 and 4 with a car, and they are just glancing blows. Finding parking takes time. Serious German wineries require reservations. We strolled public paths in vineyards in T-T.

The views from the KD boat are superior to those from road or train. The train is so much faster than the boat that recovering your car after the boat ride is trivial. Search here for some of Russ’ advice on Middle Rhine castles and sleeping.

All in all you have some very nice towns! That said, it's very rushed on the whole. And you have about 20 towns that are going to blur together in your minds once you're home, as they are shockingly similar.

Day 3: Drive to Dinkelsbuhl. Lunch at Burg Guttenberg on the way to
Heidelberg. Explore Heidelberg and stay for 2 nights. Day 4: Driving
day trip to Michelstadt, Miltenberg, and Neckarsteinach. Last evening
in Heidelberg

Day 2 (6 towns) is followed by these 2 days, which are mighty rigorous and involve some backtracking as well. I imagine you will want to do something besides drive through D'bühl, that you'll do more than lunch at Burg G. (falconry show, castle museum, etc.) So after this westward journey to Heidelberg, there'll be little time left in your day. On Day 4 you zag back in an easterly direction again for another full day outing, then back to H'berg for the night. "Explore Heidelberg" seems more a faint notion than a plan - so I question why you are staying there and making the detour just to do so.

The suggestion below is still rushed but a little less rushed, a little more doable, I think... I tossed in a couple of specific recommendations that should add a small amount of diversity to your trip here and there.

1) Train from Interlaken to LUDWIGSBURG, visit Palace in afternoon (skip Hohenzollern, etc., see the real castles on the Rhine/Mosel instead.)
2) Get car in Ludwigsburg > Besigheim etc. > Burg Guttenberg (and overnight nearby - BAD WINDSHEIM is a beautiful place.)
3) > Burg Guttenberg > Dinkelsbühl > Rothenburg (2 nights.)
4) In Rothenburg; Possible half-day outing to Bad Windsheim's open-air historical/cultural museum.
5) > Miltenberg > Michelstadt > Auf Schönburg (check in early if possible.)
6) Linger in morning. Drive into town, take the town wall walk. > St. Goar (find lunch on the waterfront - Rheinfels Castle?) Use ferry crossing to reach Braubach and Marksburg Castle (4 pm tour in English.) > Cochem (2 nights)

Day 7: squeeze in a cruise to Beilstein in the afternoon?
Day 8: I would visit Bernkastel in the morning but leave Zell and the others in the dust as you make your way to TRIER for one night. You should have time for a few sights in the afternoon and the next morning before catching your train to Paris.

A lot of driving perhaps, but we're flexible to know that if we like
anyplace that we stop we'll take the time to enjoy where we are and
make adjustments as we go along.

With your current plan, you are going to like all these places, but you will likely not have the time to explore/relax in/ enjoy them well.

I would highly recommend spending one day at the Bodensee (Meersburg!, maybe also Konstanz) which is close to Singen, because the towns for day 2-4 are fairly similar and Meersburg certainly is more beautiful than Neckarsteinach or Dinkelsbühl. I´d remove Neckarsteinach and some of the towns on day 2 to make it possible.

You could go to Konstanz from Singen, then take the ferry to Meersburg and back. It is absolutely lovely and doable in one day.

When you are on the Mosel River at Cochem be sure to visit Beilstein. It is my favorite tiny village on the Mosel. I access it by bicycle which I rent in Cochem. It is an easy ride on the bike path along the river with a crossing on the little ferry across from Beilstein. With a car it is also easy to access, but the train does not go there although the river boat does. Beilstein is so quaint and the actual village has no access for cars, but you park along the highway or street.

Thanks everyone for your input.
We're making some changes to our itinerary based on the suggestions you've made, and would appreciate any additional comments and suggestions.
Our re-worked itinerary is as follows:
Day 1: Sept. 5 - Arrive by train in Ludwigsburg around noon from Interlaken. Drop luggage at campuszwei - Hotel for 1 night stay. Visit Ludwigsburg Palace in the afternoon.
Day2: Pick up rental car. (I believe Europcar has hours on Sunday in Ludwigsburg?) Follow a route through the towns of Markgroningen, Bietigheim, Bissingen, & Besigheim, (a portion of the "Deutche Fachwerk Strasse"). Visit Burg Guttenberg then on to Rothenburg ob der Tauber for 2 nights.
Day 3: Possible day trip. Suggestions? Considering Dinkelsbuhl or Bamberg. Or just hang around Rothenburg?
Day 4: Drive to St.Goar via Michelstadt for 1 night. Looking for hotel recommendations. Explore town / towns with available time. Perhaps visit Loreley
Day 5: Train to Bingen, Rhine boat trip with stops at Bacharach and back to St. Goar but possibly as far as Boppard depending on time. Move to Oberwesel for 1 night at Burghotel Auf Schonburg.
Day 6: Spend the morning at the castle. Drive back to St. Goar, take the ferry across the river and drive up to the Marksburg Castle. Lunch in Braubach, then drive to Cochem for 3 nights. Explore Cochem on arrival.
Day 7: Morning bicycle ride to Beilstein and beyond. Visit Eltz Castle in the afternoon.
Day 8: Day trip for winery visits and wine tasting in Bernkastle-Kues and / or Traben-Trarbach. (Guided tour or self-guided / driving?)
Day 9: Drive to Trier, return rental car, and catch a train to Paris at approximately 11:40
We've considered dropping 1 night in Cochem in favor of Trier for 1 night, but ultimately decided we'd like to take the time to enjoy the Moselle river area for one additional day.
Again, any additional comments or suggestions are appreciated.