Germany itinerary review

"...maybe in Germany more than any other country, opinions on towns and sites vary WILDLY."

Maybe. Maybe it's just that there are so many good choices. Also, your ideas have shifted around a lot. We're probably all a little unsure what you want out of this trip.

"How's this?"

Your new itin. has some built-in backtracking that wastes time. I suggest doing some "hopscotching" with stopovers instead to hit your destinations...

Bruges to Cologne
Maybe arrange a stopover in Brussels on the way - but if Brussels is just an afterthought, then you could visit Aachen instead on the way to Cologne (use lockers) and thus avoid a backtracking day trip. You pass through Aachen on the way to Cologne anyway. Then I would think 2 nights is enough for Cologne.

Bonn
You could save the day trip there and back to Cologne again in a similar way by visiting Bonn (use lockers) for a few hours on the way south to Mainz (2 nights.)

Rhine River cruise and Castles
I see you've axed Boppard. OK. The day trip from Mainz you have planned means backtracking along the Rhine again - then going back to Mainz. I think after your sightseeing in Bonn, I would just head to one of the scenic Rhine towns (instead of Mainz) for 2 nights. Maybe Bingen. The Bingen NH hotel was my wife's favorite place on a recent trip. Great setting on a long riverfront promenade and a great breakfast too. After breakfast the next day, board the 9:30 northbound cruise boat and head off to your villages and castles. On the FOLLOWING morning, catch your train to Mainz, drop bags in a locker again, and see the town for several hours before heading to Würzburg for 2 nights. Train time from Bingen to Mainz to Würzburg totals about 2.5 hours. Not bad at all.

Leaving Würzburg with car in the direction of France
Sounds like you want to meander by car. OK. On your way to Calw or wherever, I suggest you detour to hit some of the towns and castles along the Castle Road. Just drive south from W'burg to pick it up.

Russ,
You've worn me down on staying in Bingen instead of Mainz... :)

June 7 - Arrive Brussels 8:45 am, stow luggage and hit a couple sites in Brussels that interest us. Train to ...
Bruges - two nights - was planning to hit Ghent on the way to Cologne, but you've suggested to do Aachen and I don't think we'll have time for both, will we? Suggest we skip one or the other, or just day trip back to Aachen from Cologne?
Cologne - two nights unless we're using one day to go to Aachen, then we'll need to keep the third night here.
Stop in Bonn on way to ...
Bingen - two nights - river cruise, etc... (may stop in Mainz on way to...)
Wurzburg two nights - rent car - Castle Road per your recommendation on way to wherever we sleep next. Etc.

Better? I'm tired already. lol

While I realize you have now erased Mainz from your proposed itinerary, I still want to recommend a couple of superb sites in that very pleasant city. First is the Cathedral/Dom, one of three great Romanesque cathedrals in the Rhineland - the other two are in Speyer and Worms - which was built in the mid to late 900's and the very early 1000's . The second site is the St Stephen church which has some awesome glass windows designed by the artist Marc Chagall. Also for book lovers the Gutenberg Museum is in Mainz. And of course you still have easy access to the river for day cruises.

Why Ochsen Post in Tiefenbronn? Really what do you want from Bayern and Baden Wurtenburg?

Calw is a cute, Avant-garde, little town, full of Fachwerk buildings and the hometown of Hermann (Steppenwolf) Hesse, A nice place to visit, but not, I think, worth an overnight visit.

Thanks all. I never quite know how to answer the question "what do you want from this?" I've never been to these places before so I may have a preconceived idea, but my questions are really my attempt to pick the brains of you who have been to or live there to narrow down what to expect and what to do.
My preconceived notion is that after we leave Wurzburg, we'll have a rental car to drive secondary roads through towns and villages, home base in a smaller town where we'll have easier parking but still a couple nice restaurants close by, but otherwise, can just spend time seeing pretty towns and site of interest. My husband is a musician and huge history buff, but we don't want to spend every minute in museums. We'll do some of that, visit a couple of castles, but like to leave some things to chance, and allow ample time for just being wherever we are. We'll sometimes be driving down the road and stop when something looks interesting and walk around for a while. I was not joking when I said sometimes that is a supermarket or some roadside stand.
I asked about Ochsen Post because it looked like a good place to sleep a couple of nights, in the general area we'll likely end up. Do you have any other suggestions?
Thanks!