Belgium, Germany, Switzerland, ...

You indicate that you will visit Spain, then France. Not sure where you plan to go in those countries, or where you have been.

I know you wish to stay in one place, but that does limit your options, since you would use that place to do day trips.

I would normally advise you take trains from Madrid to Paris and visit places like the Basque region of northern Spain, then across the border to the Bordeaux region of France, but you may have those areas in your plans already.

We have done the basing and day trip concept in England and Italy. If you are interested in the UK, we did a great four week drive tour of England and South Wales and for six nights stayed in Chipping Campden in the Cotswolds and visited Oxford, Blenheim Palace, Stratford Upon Avon as well as a couple of days in the Cotswolds.

I suggest that you not plan on traveling more than about an hour and a half away from your base. You could do this in Straussbourg, France, on the German border and visit Alsace as well as the German Black Forest (Friberg and Triberg), If you aren't doing the Loire Valley in France, you could spend 3-4 days there. We stayed in Blois and loved it.

If you aren't planning to visit Provence in France, consider taking a Rhone River cruise from Lyon to Arles. You could fly to Amsterdam and visit that great city as well as some of the surrounding areas.

Our intended pace is "relaxed", rather than go-go-go/must see
"everything"

It's a very individual thing what one feels as "relaxed", but several countries within a short time would feel rather "go-go-go" to me. If I had Spain behind me and France before me and wanted something relaxing, I'd stay somewhere in between those two destinations, like Southern France or Northern Spain as geovagriffith suggests. For that, it would help if you told us where in Spain and France you plan to go.

We'll have about 10 days to go "somewhere" between our time in Spain, prior to start of our time in Paris and other areas of France, mid-May 2022. Our intended pace is "relaxed", rather than go-go-go/must see "everything"

After looking at a map of Europe and considering your preferred items, I would suggest staying in St. Goar along the Rhein. The train runs up and down the Rhein to very interesting small villages (Bacharach, Boppard, etc.) to walk around at your leisure. Also, you can take a nice cruise on the Rhein and train to any starting point and from any ending point you prefer. There are also larger cities like Heidelberg you can train to in 2.5 hours or Cologne in 2.0 hours. The Rhein area is quite relaxing and St. Goar itself is very nice. It may be worth researching because it may just fit your plan. Also, Frankfurt is only 2 hours away and you can fly or take a train from there to Paris.

You can fly nonstop from Madrid to Munich (2h 30m) for less than $50. I would rent a car at the airport and drive to Füssen (2h 15m) and make that my home base for ten days. Dy trip options include:
Munich (1h 45m) or direct train (2h).
Dachau Concentration Camp Memorial (1h 45m) or 15-minute train from Munich.
Neuschwanstein (15-minutes)
Hohenschwangau (30-minute) walk from Neuschwanstein.
Oberammergau (45-minutes) and see the passion play.
Linderhof (1h)
The church in Ettal (1h).
Viewpoint of the Zugspitze (1h 15m).
I would buy Rick Steves Germany 13th edition guidebook.

Oberammergau (45-minutes) and see the passion play.

[from Füssen, it was suggested 45 minutes by car]

That might be a bit rough as a day trip since the performance isn't scheduled to finish until 10:30pm, 8 hours after it started. And then half an hour back to the car and 45 minutes back to Füssen in the dark?

Tickets are not easy to get, and many folks will sell packages including meals (also not easy to get) and accomodation for 2 or 3 days.

Having driven between them myself, but never at night, I don't know if that is a day trip I would choose. I'd also suggest there are better bases for several nights than Füssen and I've visited there 3 times. But to each their own.

Since Nigel pointed out that there are better places to base oneself than Füssen consider sleeping in Oberammergau instead. Day trips would have to be done by car not train. Here are possible day trip opportunities:
From Munich airport to Oberammergau 1h 45m
Munich (1h 30m)
Neuschwanstein (45-minutes)
Linderhof (15-minutes)
Ettal (15-minutes)
Zugspitze (45-minutes)

Need to return to Paris to begin time in France

Garmisch/Oberammergau and Füssen areas are pretty far from Paris.

The already-mentioned Black Forest is probably where you ought to look first IMHO.

• Lots of apartment rentals in many different small towns. Go this official tourist office page to find the widest selection and narrow your search:

https://www.schwarzwald-tourismus.info/stg/ukv/result?tt=90ba90grnaloelug653e2g55d6

Gengenbach is an oft-suggested old-world walled town - and a KONUS town (see below) - not too far from Strasbourg and a good launching pad for other outings too.

• Laid back region where change is slow and traditions persist.

• Proximity to Strasbourg, your gateway rail station for the direct TGV train to Paris, means 2 hours or less of travel time (10:53 - 12:43, for example.) Check flights to Strasbourg (SXB), also Basel / Mulhouse / Freiburg (BSL), Karlsruhe/Baden-Baden airport (FKB,) and Stuttgart (STG.)

• Numerous hiking options.

• Hubs will enjoy a day trip into the city from the Black Forest town you choose into Freiburg or Strasbourg.

• Free train transportation throughout the Black Forest Region to places of interest - just stay in one of the KONUS towns for your free host-issued KONUS card:

https://www.youtube.com/watch?v=KkgzMXQ7blQ
https://bw.tourismusnetzwerk.info/wp-content/uploads/2019/05/KONUS-Flyer-PDF-Download_GB.pdf

Freiburg is NOT one of the Black Forest KONUS towns. It is a city of some size - guess you might stay there if Hubs gets his way. But I'd prefer a stay somewhere smaller (like Gengenbach) and dangle a day-outing to Freiburg as a carrot (free trip on the KONUS card.)

Thanks very much for the informative replies. I've now omitted/shuffled/modified a few parts of our trip so that we go from Madrid to Paris, Paris to Germany (Black Forest area is winning out right now!), then Germany to Provence. Looking forward to more adventures!

I am sure you have your reasons, but wouldn't it be easier travel to do Madrid to Provence, Provence to wherever in Germany and Germany to Paris? It seems you are ending your trip in Provence. Just a thought on making travel logistics, especially plane flights, easier to secure.

Ah, yes, it would be easier but when one is using airline miles to secure business/first seats to make that journey back to the US a bit more tolerable, one doesn't always get to choose "easy". We are frugal with our miles to get maximum benefit and booked the moment seats became available, typically around 11 months out. The number of miles required to fly business/first back from Paris was beyond our reach!