Like Maildir? Many mail clients support that format already.

Yes. From experience productivity tools are never good because either:

1. They are simplistic 2. They are too complex

If I want to get anything done I want to build my own system where nobody else has a say in how the thing should behave.

I’ve build my own workout app and journaling system because after trying a trillion none of them fit me.

Today it’s easy to build tailor made programs quickly so I’d rather use something I control end to end.