If my wife and I both use Proton, or I correspond with a bank or company that doesn’t use a Google product, how would that possibly get into Google’s hands? You way oversimplified the possibilities here.
I can send you PGP encrypted email you could have used Gmail or Proton or whatever. It is about what the sender does not what service you choose. (If you and your wife are both using Proton, that probably does not have anything to do with SMTP; it would be like calling iMessage "SMS".)