[aeyes]

If you want to do upgrades like this on RDS with minimal downtime you will end up doing the same process: Set up new servers, do logical replication, switch over.

The RDS update process is a single button and you have no way of knowing how long it will take. There are some tricks like turning off Multi AZ and taking a snapshot manually before starting the process but still - for large instances you could be waiting anywhere from 30 minutes to 3 hours for RDS to finish. With large instance types I have seen RDS take a full hour just to provision an instance, in the meantime you'll sit there hitting F5 not knowing if it will ever finish.