Does anyone recommend any alternate providers with custom domains, or some OSS? Is it possible to host your own email server on a NAS or RPi something?
It's absolutely possible to host your own e-mail server on VPS. You'll receive mail without issues. But sending mail might cause issues, so unless you're OK with some delivery problems and spending some time to investigate, I don't suggest going that route.
Hosting your email on NAS is problematic. You need to have static IP address with PTR record and most home providers won't offer those services for reasonable price.
With the complete lack of accountability, support, or recourse the giants seem to have, it has never been more important to not put all one's eggs in one basket.