with email, the meta data leaking is at the protocol level, email is comically insecure and no matter funny encryption you do with pgp the protocol itself will leak data, and Proton's advertisments as a secure private email provider are misleading in a fundemental level thanks to this, I do not see how any email provider could fix this other than making a whole new standard for an email-like protocol

email is a legacy tool that needs to be phased out and a sane better replacement has to be made, untill that there is little to no hope to not leaking email metadata to some degree since email is effectively required to create accounts in most web services