I think this level of key management is relatively simple for a production application. There are not really too many pieces and certainly no unnecessary steps (the mitigation recommendation was to add more keys, not fewer).
The interesting part is really just the first step - starting with a user's password, to statically derive a service-authentication key and a data-encryption key that the service provider ostensibly never sees.
At $DAYJOB we independently came up with this same zero-knowledge solution too, using slightly different primitives. Ideally there'd be some well-reviewed zero-footgun nacl.SecretBox()-style thing for this use case, but there simply isn't.
The interesting part is really just the first step - starting with a user's password, to statically derive a service-authentication key and a data-encryption key that the service provider ostensibly never sees.
At $DAYJOB we independently came up with this same zero-knowledge solution too, using slightly different primitives. Ideally there'd be some well-reviewed zero-footgun nacl.SecretBox()-style thing for this use case, but there simply isn't.