The bridge does the decryption using credentials you give it locally. Sorry for mentioning "auth". I should have mentioned encryption instead.
Regarding the rest, it comes down to the zero access mailbox encryption's implementation details. In all described scenarios, you're not really using your master password as the "key" for your mailbox. But in proton's and similar services' case like Tuta, this is true. Any "zero access" service provider offering IMAP access without a bridge is simply lying to you as IMAP (the protocol itself) requires server-side decryption of the content, even if SMTP doesn't. (Btw, SMTP is really an artificial limitation. Just not IMAP. If they give you smtp access, it wouldn't send encrypted mails unless specifically configured to do so but would otherwise be the same.)
What you described is encryption at rest, but not zero access encryption (which is what Purelymail does btw).
Whether all this is needed and all depends on your threat model. I think most tech-savvy folks would be happy with something like Purelymail or Migadu tbh...