Now, Proton and various other “encrypted email” services then take that plaintext and encypt it with your public key, then store the ciphertext on their servers, and then they’re supposed to discard the plaintext, so that in case of a future court order, they wouldn’t have the plaintext anymore.

You would not be able to retrieve your mails using IMAP from a regular mail client if they were doing that. You can even retrieve them from Gmail, which is unlikely to support any kind of “bring your own private key to decrypt mails from IMAP”.