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. Three pods, one primary and two replicas, each on its own replica-1 PVC. Semi-synchronous replication, automatic failover by the operator. A replica that loses its place in the binary log is rebuilt from a fresh physical backup of a ready replica (replica.recovery with the mariadb-replica PhysicalBackup template). It is the same layout as Postgres, and it is the topology the operator supports point-in-time recovery on.

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