Have you considered how proton mail filters spam? It has to be stored in memory for this to happen before it's encrypted and flushed to the disk.

Unless your emails are encrypted for you on someone else's client before they're sent to your protonmail, in which case protonmail still has all of the metadata (time sent, where it was sent from, length of the message, etc.)

Do you believe proton mail can keep state level actors like Russia, China, and the United States out of their prod infra?

They may be "mostly" subpoena proof as in they can't disclose old emails, but do you trust them to not hand over your data before it's encrypted?