How to alter tables safely to set row_format as Compressed or Dynamic in Mariadb Galera Cluster?
Since you are not really changing any data nor the schema, just the underlying row_format, seems like RSU would be better, and a lot less invasive.

You might also take each node out of the cluster while doing the ALTER, or at least remove the one node from the proxy's list.  This should avoid letting queries hit the table.

I feel pretty sure that the table will be blocked from writes and possibly reads during the conversion.

It would be wise to test this on an offline cluster, including testing reads/writes.