We currently use render. Its excellent, but is missing a couple key features: like Heroku's continuous rollback protection. If they offered this they'd be head and shoulders better.
Think we (Crunchy Data) has been mentioned a few times below and have near feature parity with Heroku Postgres (only thing missing is dataclips coming this quarter). We've got folks that migrate the DB to us then use a whole host of other options including Fly, Railway, Render, etc.