@inthehands I wasn't sure where in this thread to reply, but I was told yesterday that the "secret sauce" bit to Proton (and Tuta) is that only you have the decryption key to read the contents of your email. For example, I read Tuta's security page yesterday and their email search index is on your machine, because they can't do it.
So, it's slightly better than what is obviously a protocol never intended for private communication. Having said that, I did prefer that my email was sat outside of US jurisdiction (Cloud Act) vs having to go through an international warrant.
Good thread, thanks for putting it together. Email is definitely one of those things that are old enough that people won't know this stuff. That everything is encrypted in transit is something I learned, actually.