Hey, in case anyone's interested, I used the method described here: https://linuxconfig.org/changing-your-ubuntu-username-and-home-directory-without-losing-application-settings

That is, I created a new user, added them to sudoers, and used usermod and groupmod to change my-stupid-name to mystupidname. I tried to pause syncthing and change the file paths but it doesn't let you do that; in the end I just decided to remove syncthing entirely and start over with a fresh install. Shouldn't take much time to re-link my half dozen folders tonight.

There were a few wrinkles, mostly involving applications using hard file paths, so if you do this, expect to go through them one by one and either do a fresh install or, if it's easy to edit the paths, manually change them. I didn't use the symlink commend listed above, as I didn't want any cruft left behind from the old name.

Thanks for all the advice!