I would not use the word "base" for Zurich in the itinerary you posted. Except that you have relatives there, there is absolutely no reason to return there. As first-timers to Europe, you may be unfamiliar with the term "open-jaw" air ticket. And the time and money spent on the return are better used seeing other cities. You also may not have learned yet whether you prefer to take short daytrips in the area of a big city, or change rooms often (like Rick's tour books often do) to see many more cities. It's a matter of temperament and preference.
You also need to do a little more research before you firm up the itinerary. Even if your husband's interest is, say, modern architecture and modern ( ... not "contemporary"?? ) art, I see little reason to schlep to a fading post-industrial city that has nothing but the Guggenheim Bilbao. Even Santiago de Compostela has "more" to see and do than Bilbao-to pick another slightly isolated destination. In fact, you could go to the Vitra Design Museum campus (steps from Basel, Switzerland) and see a Gehry building from the outside, and tour every inch of Zaha Hadid's first constructed project, the Vitra Fire Station. (You could also visit a Gehry building in several U.S. cities.) But Switzerland, Germany, and France (among other countries) are loaded with a staggering quantity and quality of both old and new art museums. The Fondation Beyeler (in Basel) is among the most important modern art collections in the world. If only they had more display space.
I don't mean to pick on your husband, but I think you have not done enough preparation to use the three weeks to best advantage. Have you bought (or read at the library) Rick's introduction to travel books, (like Mona Winks, for example), and skimmed the books for the individual countries you're considering? Have you checked how expensive your time in Switzerland is going to be - even if the room is free?
I'm 63 myself, but have traveled enough that I don't find getting to the train station very stressful anymore. But you mentioned your desire not to spend too much time on train travel. The first trip we took to Europe was a week in Paris and a week in London, with local daytrips (like Versailles and Greenwich, for example.) Maybe you should think about closer together locations and fewer countries? Although I hate going through Heathrow, we went home from Santiago de Compstella because there are so many flights from Northern Spain to London. Could you interest yourselves in big cities that are well-connected? Look at a map of just Thalys trains, for a start. I mean like London-Brussels-Cologne-Amsterdam and home. Or if you have to include Zurich, London-Paris-Brussels-Cologne-Basel-Zurich. Did you give the month of travel yet-are you interested in mountain-Switzerland?
Another issue is whether your budget will permit only one trip to Europe in your retirement. That would justify a lot more time on trains.