How to restart MariaDB Galera cluster to change my.cnf
rsync is usually not a good SST method as it can block writes on the donor node while performing SST. Use mariabackup instead to avoid that.
Use wsrep_provider_options="gcache.recover=yes; gcache.size = xM;" where x is enough megabytes to hold all write sets for any expected downtime, if you want to be able to use IST and avoid SST after restarts. E.g. the amount of downtime expected for upgrading MariaDB on a single node. 128 is what I use, but it obviously depends on how much writing is usually happening on your cluster. gcache.recover=yes; is needed to try and use gcache-based IST when a node starts up instead of SST. See the MariaDB KB article on wsrep_provider_options for details.