Replacing a $3000/mo Heroku bill with a $55/mo server

It sounds more like poor choices. 6 staging environments sounds a bit overkill.

If you can fit them all on a 4 cpu / 32gb machine, you can easily forgo them and run the stack locally on a dev machine. IME staging environments are generally snowflakes that are hard to stand up (no automation).

$500/month each is a gross overpayment.

> you can easily forgo them and run the stack locally

Not if you're running with external resources of specific type, or want to share the ongoing work with others. Or need to setup 6 different projects with 3 different databases at the same time. It really depends on your setup and way of working. Sometimes you can do local staging easily, sometimes it's going to be a lot of pain.