As a Fastmail user (that's perfectly capable of managing his own PGP keys), what features am I missing? I honestly can't think of a single feature that'd make me go "I wish Fastmail had that".
If you're not using your own domain, and are instead using the provider's domain for addresses, one huge difference is that Fastmail recycles deleted addresses within a few months, whereas ProtonMail never recycles addresses on its domains. That to me is critical.
How about labels that operate independently of folders like Protonmail has?