I’m going specifically after Proton because of the fact that their PGP encryption doesn’t provide any additional value that users lack

This is not exclusive to Proton.

even when using Gmail you can send your mails with the same PGP encryption

As you already mentioned, Proton just makes this easier. Which is an entirely different discussion from how it's supported on the other end. Also criticisms of PGP are not Proton's responsibility or fault. None of this is specific to Proton.

All while with their services removes one of the only good things about email which is the ability to use your own client

Not necessarily, they created and published the Proton Mail Bridge. But this is an inherent limit of encrypted data. Once again, nothing to do with Proton. Tuta and Mailbox don't support this either.

using email with rendering enabled is just a horrible for privacy either way.

See previous response about doctor's appt.

Would be awesome honestly if we could replace email with Matrix

Something something candies and nuts