Hi!
have about 60  RB951
all in a single network via VPN

there are several administrators serving all

very problem each time to change usernames and passwords on all routers

maybe there is a possibility to use an external server containing usernames and passwords?

Sure!
Create a RADIUS server somewhere and configure authentication via RADIUS.
This can even be a Windows domain controller with the appropriate extra package.

sorry
was blind
elephant did not see