An unrestored backup is an unverified assumption.
Every managed database has a native backup mechanism — only a real restore drill confirms it actually works.
Regional failover
Synchronous replication to a standby zone — automatic promotion on primary failure, brief connection interruption
CMEK
Your own Cloud KMS key replaces Google-managed encryption — revoke the key, and the data becomes permanently unreadable, including by Google
A destroyed CMEK key is genuinely irreversible data loss with no safeguard. Treat key management with at least the same rigor as the database itself, including its own recovery plan.