I should have mentioned that my phone is a Pixel 2 xl and doesn't support non-Google eSIMS. But I was also forgetting that I probably want a French phone number, which means either a new SIM or gritting my teeth and using the Toggle card I've been keeping alive for emergencies. Hmmm.

Thanks for the suggestion and for sharing your Project Fi experience.
[EDIT] Looks like Toggle is out. They no longer offer data outside the UK except at the extortionate price of £0.15/Mb -- US$1000/GB!