I was wondering if it would be possible to only force SSH key based authentication to one user (admin) and allow all other users on the system (excluding root, who's ssh login is already disabled in sshd_config) to authenticate via Password.
You can do that by modifying your
sshd_config to add:
Match User <username> PasswordAuthentication no
You can do the same for groups as well by using
Match Group instead
Don't forget to restart