I don't think it's true that it works best by doing bottom-up. I develop top-down all the time.

Whenever you need a new function/method/struct field/enum variant, just write its name where you are going to use it. Then intellisense will complain that it doesn't exist. Press Ctrl+. (Or whatever you have keybinded to "apply suggestion") And now it suddenly exists and intelligence works perfectly fine. It will just place a todo!() or raise UnimplementedException or whatever so you don't forget to implement it later.