galera cluster how to run an ALTER without using RSU/TOI
RSU:  You must bypass the load balancer and go straight to each node in turn.

TOI:  Simply run the ALTER on any node.  The downside is that if the ALTER takes a long time, it will, as you say, lock/stall the cluster.

gh-ost:  This depends on the binlog, which (I think) is not used in Galera.

pt-online-schema-change:  This requires adding TRIGGERs to the table -- perhaps a problem?  And it requires a script to continually talk to the server in question -- will your load balancer allow such?  And it probably wants to treat the system as Master-Slave(s), not Multi-Master -- apparently you have set up Galera set up Multi-Master?