RDS DB Instance crashed with no prior metric warnings — seeking root cause

Based on the pattern you've described—stable metrics followed by an abrupt crash with recovery events—this does suggest a potential underlying infrastructure issue rather than a resource exhaustion problem. When CloudWatch metrics show no gradual degradation and all metrics drop simultaneously, it typically indicates an event at the host or hardware level rather than an application or database-level issue.

Is there any way to confirm a host-level event outside of opening an AWS Support case?

No, AWS Support has the required tools to confirm the accurate root cause for you, if you want the formal RCA.

Any recommendations to reduce downtime in similar scenarios (e.g., Multi-AZ, RDS Proxy)?

For production workloads, implementing Multi-AZ deployments is highly recommended for high availability. Multi-AZ provides automatic failover to a standby instance in a different Availability Zone, which can significantly reduce downtime during infrastructure events. In a Multi-AZ configuration, if the primary instance experiences a hardware failure or needs to restart, RDS automatically fails over to the standby, typically completing in 1-2 minutes rather than the 9 minutes you experienced.

Since you've already opened a support case, that's the right path to get a definitive root cause analysis. The support team will have access to internal logs and infrastructure telemetry that can confirm whether this was indeed a host-level event. Once you receive their findings, implementing their recommendations along with Multi-AZ (if not already enabled) will help prevent similar downtime in the future.
Sources
Troubleshoot Aurora PostgreSQL restarts and failovers | AWS re:Post