I have a limited amount of experience with carbide, as in cutting tools. Carbide is extremely hard and tough, and can be ground to a very smooth finish, all of which makes carbide inserts for sizing dies nearly perfect. Any machining (or sharpening) is done with diamond impregnated cutting tools, special methods and coolants for cutting/machining. All the carbide I've seen comes in chunks of varying sizes and shapes and are "drilled", shaped, and finished ground with diamond tools, much like glass. My guess for a 9mm full length, tapered sizing die of carbide would cost several hundred dollars to produce.
Not arguing, just sharing my ideas...