I actually don't use protonmail but you should be able to access your mail with IMAP (both iCloud and Gmail support it so proton should as well).

If by encryption you mean PGP etc, that's handled in roundcube I believe.

mbsync mirrors your protonmail inbox to disk, dovecot shares that mail folder through IMAP, and roundcube or some other frontend connects to dovecot.

For gmail/icloud IMAP, you can't use your regular account password, you have to create an "app password" and use that password for mbsync.