> That sadly immediately rules out Protonmail.

I think you could run the ProtonMail Bridge on a personal server. It wasn't intended for that, so IDK how secure it is for exposing on the open internet.

Protonmail is soooo good, so I'd hate to rule it out, and I'd like a solution too. And it makes sense why they don't support IMAP directly, because the protocol doesn't support it, because the emails are all encrypted on their server, and IMAP doesn't support end to end encryption (to my knowledge)