Galera wih MariaDB master-master to support write enable on disconnect
First, Galera should not be used for more than about 5 nodes.  This is because each node talks to every other node on each write.  This means 100 nodes would have 9900 'connections' among them.  And each write involves 99 roundtrip messages.

You have solved the main 'key' problem by ensuring uniqueness.  But there are other possible ways of having conflicts.

Furthermore, Galera's approach to re-acquiring a node will not work for you.  It will assume the node is corrupt.  It will not assume you have inserted valid new records.

Your dual-master setup does not affect what will eventually happen on the Galera nodes.

Are you more vulnerable to server outage?  Or network outage?