I recently left ProtonMail and went back to Fastmail. My reason was that they will never be able to fully support IMAP and now CalDAV because of the encryption they use. I grew to accept that email is not for secure messaging and my paranoia of "I'm being watched" just went away. If you need secure messaging, use something other than email.
https://protonmail.com/bridge/
Or did it not work for you?