I think one of the reasons LTN has two separate signals is for stations that request both fluids and items, since, like you said, you can't set request thresholds per item. If you set both the "request threshold" and "stack request threshold", the regular threshold is ignored for items but not for fluids, so fluids use that request threshold, while the stack threshold is used for items.
I think a map setting would work, I'm just not sure how it would interact with fluids trains. I think a separate signal would suit me best, but if necessary I can make due with per-item thresholds.