The stack per engine

Postgres — CloudNativePG. Three instances, one primary and two replicas, each its own PVC on the replica-1 class. Streaming replication, automatic failover in seconds, a lost replica rebuilt from its peers. The operator makes the three-instance layout nearly free to run.

MariaDB — mariadb-operator. Galera for synchronous multi-master across the three nodes, which maps cleanly onto a 2-of-3 quorum; any node serves. Galera's cost is real — it dislikes large transactions and DDL, and carries SST/IST mechanics — so async primary-replica with operator-driven failover is the lighter alternative where zero-downtime writes are not required. Either way, replica-1 storage under each instance.

Storage HA and backups are orthogonal: the choice above is unaffected by how backups are taken.