Just wanted to follow up on this because I've been looking for a similar solution.
https://github.com/kristiandupont/kanel lets me choose schemas and map custom db types and enums to primitive types.
It requires your DB login details (as it goes directly to Postgres, rather than using the REST endpoint), but it generates the interfaces directly from the tables.
I'm not sure if it types RPC functions or not - definitely worth investigating though