How to backup a Galera node
This would work, although you might want to consider a physical backup solution with percona-xtrabackup (innobackupex) or the new mariabackup. Depending on the load on your cluster, you can even do these physical backups directly on one of the cluster nodes. (Though in my experience, xtrabackup does lock (FLUSH TABLES WITH READ LOCK) for about 15-20 seconds during backup of our 30G database.)
I don't have experience with this, but it sounds like it should work, obviously provided that when you restore from this backup you bootstrap the cluster from this node.