Galera cluster node fails with error. Brand new cluster + master / node. Access denied or can't connect
It doesn't sound like the problem is related to Galera. Maybe the debian-sys-maint@localhost user had already been created with the unix_socket as the defined authentication plugin before you did the GRANT for it? And the error message is pretty clear on what is the issue with that: ERROR 1524 (HY000): Plugin 'unix_socket' is not loaded.

So, you can either recreate the debian-sys-maint@localhost user with the default authentication plugin - mysql_native_password (option 1), or you can install the unix_socket plugin (option 2):

Option 1

DROP USER debian-sys-maint@localhost; 
CREATE USER debian-sys-maint@localhost IDENTIFIED BY 'your password'; `
GRANT ALL PRIVILEGES on *.* TO debian-sys-maint@localhost WITH GRANT OPTION; 


Option 2

INSTALL PLUGIN unix_socket SONAME 'auth_socket';


The advantage of option 2 is that you don't need a password for the user, assuming debian-sys-maint is the name of your OS user. MariaDB will use that OS username as the MariaDB username.