You'd be surprised to find out that ProtonMail uses Google's push service (unless you have GMS disabled) so your emails aren't hidden from Google as much as you think.
your emails aren't hidden from Google as much as you think.
You're wrong, Proton encrypts notifications before sending them to you through Google or Apple push services. The notification is then decrypted in the Proton client on your phone, and only then displayed to you.
Sending plain text notifications is impossible for Proton btw, since they don't hold the encryption keys to your email inbox and can't read your emails themselves, and thus can't send you a notification with the email subject and body.