Thank you, everyone, for all the thoughts and tips. I just booked my flights a couple hours ago and am so completely jazzed now that I have exact dates worked out and know the rough outlines of my trip.
I had been messing around trying to find a deal on a RT fare somewhere, but after working out transportation to and from the cities I wanted to visit to make the most reasonable RT options work, I finally decided to book an open jaw ticket and am so relieved now that I no longer have to worry about accommodating inconvenient flights.
The thing that I am currently most thrilled about is that I am flying in to CDG and will be able to spend two nights in Fontevraud, a place that has pretty much been at the absolute top of my bucket list forever but that I was just not able to manage the last (and only) time I was in Paris. I left one night free between Paris and Fontevraud. I may just overnight in Paris and gorge on macarons, but if anyone has suggestions for another city on the train route between Paris and Saumur (including Saumur) that would be worth an overnight trip, I'm all ears. I haven't done any research yet but of course will.
I need to be in Madrid on the fourth day after I leave Fontevraud. A number of friends have recommended I visit San Sebastian. Thoughts on that or on other options also appreciated.
After my tour of Spain, I will still have the six days to get from Barcelona to Warsaw. I had originally thought I'd do Bruges and Amsterdam in those days, but I have now added a few days to the end of my trip that I can devote to Amsterdam (and I will be flying out of Amsterdam), so my thought now is to take the train from Barcelona to Switzerland and do a couple nights there (Bern? Geneva? Basel? I'm not super keen on Zurich for some reason, but it's not off the table) and then take the train to Belgium, spend several days there, probably based in Bruges (maybe Ghent), and then off to Warsaw.
I am booked for a bit after Warsaw and will end up in Berlin, so I will probably take the train from there to Amsterdam and then spend three full days there before flying back home on the fourth day.
And then I will sleep for two months!
(I hope this thread hasn't, um, gone too far off the rails in terms of where it started.)