Sorry that happened! I think it should still boot - maybe connecting a hardware keyboard, Ctrl+Alt+F3 allows you to login and undo your changes.
My recommendation is generally not try this without ssh enabled (so that you can log in to fix things) - or, even saver, to boot the Librem 5 from SD card (into PureOS, Mobian or postmarketOS) and experiment on that system.
I have tried using wvkbd with Phosh before on postmarketOS and it was a rather crashy experience - I never tried to debug this, so fixing the crashes might be easy.
To find out what to do, here’s what changes in /usr/share/applications/sm.puri.OSK0.desktop when changing to phosh-osk-stub (installable from the Octarine repo) via sudo update-alternatives --config Phosh-OSK:
See the diff here: https://media.linmob.net/diffs/sm.puri.OSK0.desktop.diff
Changing the exec line to the binary you want to run instead (I’d suggest launching it manually beforehand) is the most important change here and should do enough. I recommend making a copy of the file before changing it so that you can easily go back to using Squeekboard.
Good luck!