The issue is that the open protocols IMAP, CalDAV, and CardDAV weren't built with E2EE in mind. This is the big reason why you can't use 3rd party calendar clients with Proton.
If that's the only issue it should be trivial for Proton to release a slightly modified version of Thunderbird that allows decrypting an encrypted CalDAV XML. That could then be the basis for extending the spec to allow encryption.
