I don’t have a Fairphone but this thread reminded me of relocking bootloader on an Essential PH-1. I discovered that just the one locking command was required: fastboot flashing lock
This is despite having previously unlocked critical as well.
I saw on Fairphone support page that they suggest the same method.
EDIT (a bit late, sorry!):
Revisiting the Fairphone support page I see that I made a mistake and must have not read the article properly. Fairphone do indeed specify two stage locking; just the other way around from e’s install instructions
Fairphone: fastboot flashing lock followed by fastboot flashing lock_critical
eOS: fastboot flashing lock_critical followed by fastboot flashing lock
Begs the question: Which is right or does it even make a difference?