Hi Nirbhay,
As discussed on IRC #mariadb, you do not really need to take the node off cluster (3). Just set wsrep_osu_method's session value to RSU and perform the schema change. With RSU mode enabled, the node automatically desyncs itself from the cluster before executing any DDL,and thus other nodes in the cluster are not impacted.
I don't think this was the case during our testing, but I'll test it again anyway. If I understand RSU mode correctly all it does is ensure that DDL statements aren't replicated to the other nodes. AFAIK it won't stop the write events from the other nodes from being synced to the node in RSU mode, meaning that if we have writes to the table being altered we could run into problems. We have some very large tables and one of them we tested took around 7h 45min to add a column. Mark