Self hosted email is its own can of worms. I wouldn't recommend it to anyone outside of experienced IT people. You'll end up blacklisted before you send your first email if you do anything wrong (and there's a lot that can go wrong), and it doesn't solve any security problems email has.

Anything sent over email just isn't private. That goes for Proton customers when they send or receive anything from a non-Proton address too. The one thing privacy email providers can actually do is keep your inbox from being scanned by LLMs and advertisers. That doesn't prevent the inboxes and outboxes of your contacts from being scanned, though.

If you use email, the best thing you can do is be mindful of what kinds of information you send through it. Use aliases via services like simple login or anonaddy when possible. Having a leaked email is a security vulnerability. Once bad actors have your email, they now have half of what they need to breach multiple accounts.