I’ve been using Proton mail for almost a year now. For the most part, it has been great. I’ve thrown myself into the Proton ecosystem, using Pass, Calendar, and of course, Email. I also had tentative plans to move my VPN needs to Proton instead of re-upping Mullvad in the fall. On one hand, the ecosystem is fantastic. It all works well together, and does almost all the things I want it to do.
I do this (self host mail, have a script to encrypt incoming unencrypted mail) and have not had problems. There are lots of guides online; here’s a good one: https://www.linuxbabe.com/mail-server/setup-basic-postfix-mail-sever-ubuntu