They support IMAP. Which means, IMAP client can read your mails from the server.

Proton mail does not support IMAP. Because your emails are encrypted on the server.

Again, unless you add a layer of encryption (assuming the recipient understands it, too), it’s plaintext. On the servers.

Protonmail doesn’t claim that non-protonmail email is end to end encrypted. Any emails sent to a regular email without third party encryption will be plain text through the SMTP server, but they don’t store it. So in this case they are still not storing your emails in plaintext. Your recipient will, but that’s out of Protonmail’s control.

shows up in full plaintext on their SMTP server. Whatever they do after that (and we’ve established it’s not client-controlled encryption), they have access to it.

You’ve not established that at all. Protonmail stores that message with client side encryption and they have no access to it. Have you found any evidence that they don’t?