One key aspect that you seem to be missing is that Proton encrypts every mail, including those sent by or sent to unencrypted providers using your pgp key before storing them on the server. This isn’t a case scenario that can be handled without using a bridge

Yes it can, and I explained how. Maybe you’re the one not understanding how Proton actually encrypts emails sent by unencrypted providers/people…

In asymmetric cryptography the public key is used for encryption, then the related private key is used for decryption. This means the server just has to know your public key to be able to safely store incoming email from unencrypted providers. The Thunderbird that has your private key can decrypt the e-mails later on. This is exactly what Proton does but the decryption part is handled by the bridge.

There’s guide here explaining this in detail and providing an implementation example with Dovecot. This can be also done when a message is received by the MTA (before it is filed / stored by Dovecot) like discribed in this guide for Exim here. The process should be the same for Postfix.