I've spent the day revisiting this issue, and, after some research, I think my next phone will be Fairphone 5 with Ubuntu Touch. It's not perfect — too big for my taste and not waterproof — and I'll need a couple of weeks of trial use before fully committing. However, I really believe it could be not just my phone for the next five years but also an ad-hoc mobile workstation running a more or less usable Linux.

If anyone has experience with this setup, ideas, or suggestions, I would be glad to hear you out (if you are here after the comment section is closed, drop me an email, address is in the bio).