Wait... okay, I think we're talking about two different things.
Emails you send or receive are not private. End of story. That's nothing to do with the provider; they're just not. SMTP is from the stone age of internet when nothing was private, and the attempts to graft a layer of encryption on top of it are from the bronze age, when encryption wasn't very standardized or well-tested against real threats, and all of that shows. Even if you put a significant amount of work into grafting full end-to-end PGP encryption on top of the best your provider can do to keep your emails private, it doesn't work. Emails are not private.
What I assumed you were interested in was in separating your non-private collection of emails from your real world identity. Proton + Tor will do that, bang on. If you're trying to send and receive messages which are genuinely private, use one of the fairly good options which can do that (Signal or Matrix maybe). If you're trying to send and receive your non-private emails without it being linked to your real world identity, use Proton + Tor. If you're trying to send and receive SMTP emails without people being able to read them, you need to rethink what you want, because you're not going to be able to get that.