The option to install postgres on three instances, specify that they're in cluster "foo" and then it just works, including automatically fixing any issues when one of the instances drops out and rejoins.

That's what other DBs have but it seems to be missing from postgres. If it now exists could you point me to the doc explaining how to do this?

Stolon or an equivalent way of automatically deploying on Kubernetes with an Helm chart being officially blessed by the PostgreSQL team and made part of the official distribution.

Also, same for a multi-master solution.