Nobody ever talks about S/MIME, but it's the corporate version of PGP/GPG for mail. Apple made it dead easy to use S/MIME encryption. Most vendors do, because it's still a requirement for some government purchasing (DoD is moving away from it). I was honestly and pleasantly surprised how easy it was to use S/MIME with the built-in mail programs on macOS and iOS/iPadOS, and I'm a bit surprised that Apple didn't just automate an S/MIME key for every iCloud mail user.

What's the new standard that the DoD requires for secure communication with contractors?

Does Proton allow you to use any email client? Last I checked IMAP and SMTP is disabled and you're captive in their webmail or official client unless you pay for their bridge software.

Which makes this post ironic https://proton.me/blog/what-is-an-email-client

I use Thunderbird to access my Proton Mail. They have an app called Proton Mail Bridge that allows you to access it via IMAP or SMTP.

https://proton.me/mail/bridge

Edit: Missed the paid part in you message. Yeah, I have a paid account.

And what's stopping the government from forcing proton to hand over private keys or else?

There's no reason Chat Control can't mandate scanning in email apps, either.