This is what I was actually thinking of, not the Guard feature. It’s supposed to encrypt your entire mailbox but it also works with third party clients with some extra setup.

Also custom domain if you don’t need anonymity (using it to track spam) and want to be able to easily switch providers

Riseup and Disroot are run by collectives, Tuta uses a similar business model to Proton, Mailbox.org + Guard looks decent as well and more open than either Proton or Tuta