Just posting my solution to this, in case someone else wants it. I'm using stack inserters to efficiently throw away excess from huge platforms.
Features:
A single constant combinator that defines what's supposed to be stored on the hub, and how much of each item is to be stored
Any excess above the numbers configured in 1. is thrown away by the inserter
Any negative number configured in 1. will throw away all of that item
Items not mentioned in 1. are ignored
The rightmost circuit block is my sticky selector, which picks the biggest value (the waste product we have the most of, in this case) and sticks with it until it reaches 0, then picks whatever is biggest at that time, ensuring prioritization, as well as efficiency at the other end, since they're dealing with the same item type
Any signal not present at the hub is not present at the output, allowing stack inserters to drop less-than-full stacks on the waste belt and pick up the new item - it never gets stuck
(the screen is a substitute for the hub connection)
