But isn't this exactly what the Protonmail bridge is for? I don't use Proton myself (self-hosted Mailcow) but afaik Imap doesn't support public keys/PGP the way Proton is using it, hence one needs the bridge to use normal Imap clients like Thunderbird.
Sure, but this is only for mail, not even calendar.
Also, they do not push on flatpak which limits availability.
Contacts cannot be synced on Android.
Drive is nonexistent on Linux and I do not think it is a work in progress project yet.
Drive cannot sync folders on Android either.