The nature of the limitations AWS impose on the administrative database user account, especially because they are focused on an individual database rather than the cluster.

Shared or not is absolutely an assumption on my part. If they're spinning up an individual cluster for you, then some of the restrictions on RDS don't make much sense to me. They make more sense if it were shared.

Unfortunately, for most of the clients I work with, the limits (shared or not) are show-stoppers so I've only dealt with building a system backed by AWS RDS/PostgreSQL once and the need to dive into the underlying mechanics just wasn't there at the time.

I'm pretty sure from my experience it's not a shared instance - you can create multiple databases easily.

The limitations are presumably so that they have control over settings for things like replication and their features that you can't mess with.

Indeed, not only can one create multiple databases on one RDS instance, but it's also OK start an RDS instance with no user database at all, and create it later.

We're heavily using multiple databases on a single Postgres RDS. Yes, occasionally it's annoying not having a true superuser, but now they've improved it so you can still have 'fake superuser' reserved connections it's ok.