Well alrighty then! I think I tried a setup like this once using a redstone comparator to measure whether the queue hopper was empty and the other hoppers always seemed to feed in two batches before the redstone signal stopped them, but this time testing in my creative world I used an item-o-meter on the queue hopper feeding the smelter and one hopper feeding into the side, then another hopper on top with 3 hoppers on their sides all redstone wired to the item-o-meter and set to output exact stack sizes in the right proportions and it’s all working out quite nicely. I’m not sure its even needed, but now it appears that fluid pipes and drums get a bonus to temperature handling ( they didn’t used to ) so a quad invar fluid pipe between the smelter and the mixer works to add a little extra buffering capacity and the system is working like a charm.
It’s a bit complicated to set up, but I think it is more fuel efficient than using the multiblock steel crucible and requires absolutely no supervision!