Rust already has most of what I want from error handling: explicit control flow, errors as values, and concise propagation with ?. The friction comes when deciding what to put in the error half of Result. We often end up choosing between precise types that require boilerplate and convenient types that hide which errors can occur. But precision and convenience do not have to be competing goals. Error types should compose as easily as the functions that return them.
While I agree with you that anonymous sum types are useful, the language designers tend to avoid anything that might include a hidden cost (in this case either the size of the enum or cost of boxing). Something along those lines might end up in the language eventually, though.