Why is it not a good idea to use MariaDB's extra_port for replication?
Replication do not need some special ports and can use the common 3306. An extra  port is a port that can be used by root as back door if there is no access through the common port.
Say if you have max_connections = 100 and your app is already used all that 100 connections, only one extra connection can be used by user with ALL PRIVILEGES granted. If someone is already connected as DBA you have no chance to connect to the server. But you still can connect via extra_port cause it has no max_connections limit. And that is the reason why you should not use it for any other tasks except an emergency access. With no max_connections set to the reasonable value you can easily exhaust your host's RAM that can hang your DB or even a host in a pretty bad way.