Protonmail allows you to add PGP key. Not sure if the user just sees 'garbage' data inside the email but it's entirely possible to send E2EE email already, just encrypt the contents of the message and send that across.
Do you really want to hand over your private PGP key to 3rd-party company? I never ever won't do that. If I will use PGP key for web email service, it is only when the service provider gives a way to communicate with my local machine so that the email text is SIGNED IN MY MACHINE and send it back to the email provider, then send to the recipients. For encryption, it can be done with public key of the recipients.
If the person has the key, they can decrypt it.