Mostly thinking out loud but looking for the people who might have the most interest in something like this.
I started mocking something up here https://github.com/asim/aslam
[1] https://umbrel.com/
For reliability, self hosting a server that will always be available is hard. Not getting an important email because your server went offline is a massive bummer.
Similarly, while filtering spam on your end might be doable, self hosted email servers are often filtered by major email providers for looking like spam.