they don't support IMAP
They don't support IMAP because they want emails to remain end-to-end encrypted, and IMAP doesn't have any way of doing that. The gateway decrypts the emails locally, then serves them as plain text.
We need something better than IMAP, that's designed for modern use cases. Something that's not stateful... Maybe a web service or something like that. JMAP seems promising but barely any providers have implemented it.