It's not just the inbound to proton.   It's also the proton to you.  They don't let you use your own PGP key in an external client.  The private key is in their web app.  Please check the podcast

Also your criticisms of self-host are not true, we've used many providers that aren't IP blocked or spam labeled.  In fact, for the setups we do, if you're stuck in spam, we'll give you your money back, so we guarantee it.  who would guarantee that if it was not possible?