In moving from cPanel to Virtualmin, any pointers/resources on best practices to move existing databases (MyISAM engine) from
cPanel MariaDB 10.3.39 to Webmin MariaDB version 10.5.22 ?
There is no such thing as “Webmin Mariadb”, there is just “Mariadb”. It’s the one provided by your OS.
And, the process of migrating is always a dump and restore. But, if you make domain backups in cPanel, you can migrate those to Virtualmin using the Add Servers->Migrate Virtual Server page. We support importing cPanel backups, which will restore the database(s) from the dump in the backup.
P.S. Tested and works fine on a dev machine, one command I had a issue with was mariadb-upgrade it needs a username and password so use mariadb-upgrade -u root -p