[dub]

Hypothetically it should be possible to make an entirely new RDS cluster as a replica at a new version and fail over to it, with a similar error rate to a normal replica failover.

Setting up the infrastructure to manually manage your own cluster failover would kinda go against the spirit of using RDS and letting AWS manage infrastructure for you, though.

[guiriduro]

Yes, its an obvious gap in their offering. I assume they wrap pg_upgrade in their RDS upgrade process, but of course it is in-place and requires downtime. Their multi-AZ replication is a high-availability solution but the primary and secondary must both be the same version of PostgreSQL, useless for major upgrades. For our upgrade 9.6 -> 13.4, we had the added complication that we used PostGIS which added a kink to the upgrade path. A reliable, caveat-free, simple AWS-provided logical replication solution for zero DT cluster upgrade was sorely missed and the downtime on the recommended path was painful.