Solution: FetchMail + Dovecot. Just need to set it up, but it's pretty much what I was looking for.

The goal is to allow easily moving away from an email provider e.g from protonmail to tutanota or fastmail or whatever. How do people achieve this?

I just want to have myname@mydomain, the emails to go to whichever managed email service that allows it, and to then grab everything from that service with POP to then self-host a proxy that multiple devices can connect to. STMP can go either to my hosted server or the managed host, doesn't matter.

The idea is explicitly not to do the job of a managed email service. No DKIM, no SPF, no DMARC, none of that.

Distro is NixOS, but can adapt any instructions given. Mentioning just in case somebody already has a nix configuration with this setup.