It's not easy to do. You need so use some kind of Latch
to switch between the wait and fill states.
For this specific problem, it's easier to just middle-click the cargo wagon inventory and reserve slots for each item.
But can it be done? I don't mind a little work to get it just right. I am just mainly trying to get the circuit network understood. What makes what toggle, what makes this do this and so on. Even if it is a scribble on a napkin that could explain it would be awesome just so I could understand how to make it work.
But thank you DaveMcW for the response! I do appreciate feedback of any kind.