> dirt simple managed Postgres

Heroku PostgreSQL is very simple, yes. But once you need non-trivial scale it's expensive and extremely non-performant. Even a medium-sized RDS will outperform Heroku's most expensive database offering by 20x in my experience. My company doesn't even run PG on Heroku anymore. We have a VPC/Private Space connection to AWS Aurora because the cost/performance difference is so extreme.