I want as little Google as possible on my Fairphone 4!

[Volker]

It is the same on the FP3:

Fairphone 3 Keyboard doesn't show up on lock screen

Welcome to the community forum. Yes, alternative keyboards have been found to not work for unlocking with a password in Fairphone OS. You can avoid this problem by using a PIN (or pattern I guess) to unlock, this wouldn’t need the installed keyboard. Although, if I didn’t miss or forget something, this has been found out and only reported so far by users here in connection with uninstalling the preinstalled GBoard. Did you uninstall GBoard? Yes, this should work via a USB-OTG adapter.