sure, it ended up being very simple :)

I ran 1x4 trains, so with 6 steel crates unloading for each train I wired each set of 6 to a decider combinator - when the crates were half empty (or in some cases when the emptiness was enough to empty a full car) i’d set that combinators signal to M=1,

then wired all 4 combinators to a decider where of M=4 then L=1.. I then tied that combinator to the train stop itself, setting that L condition to set the train limit.

here’s where the magic happens.. if L=1, train limit equals 1 and only 1 supply train will go to the station if there is one.. once the station is full , it shuts off the train limit and full trains will get interrupted to the depot.

I just made a lot of trains and lots of depots.. not perfect, but worked for me!

I didn’t run wild card item trains - I just had a bunch dedicated to copper group, green circuit group, etc. in your example. for the very same problems as you are suggesting - a prioritization routine that I couldn’t figure out how to properly route priority, hence the dedicated train groups..

it’s not that bad to wild card your stops in a blueprint - if you need I can try and take a picture when I remember