MariaDB Galera + Remote Backup
Central backups occurring on a replica is a decent strategy.
The guide on quick glance looks ok. Ensure that your server-id is unique. Per mariadb galera guidance galera cluster members shoudl have the same server-id in most scenarios.. Its unclear why binlog-format=mixed when galera requires row so I'd just keep row.
'log-slave-updates' is the critical thing of the master node but for consistency its easy enough to have this on all galera nodes in case you need to change master at some point.
Like all backups consider the process that a restoration would take and business requirements on MTTR (mean time to recover). And test the restoration process when you've got backup to see it works and how long it takes.