I do this a lot too, it works great. For any non-trivial work, we don’t really consider a design doc “good” unless the author wrote a proof-of-concept demonstr

I do this a lot too, it works great. For any non-trivial work, we don’t really consider a design doc “good” unless the author wrote a proof-of-concept demonstrating the feasibility of whatever approach is being advocated, specifically focused on whatever is novel or risky with the project. Iterative development starts before the doc is ever reviewed.