TLDR: Please help me build a smelting array that doesn't get jammed with single pieces of stone / iron plates in the furnaces.

I'm playing a ribbon world and I've designed an industrial smelter, where a train full of ores pulls in, everything is smelted, then the same train leaves with all the finished products.

The problem is that furnaces get jammed with single stones or iron plates when I'm trying to make stone bricks or steel. I need all furnaces to empty completely so that they are ready for the next train which may hold different materials.

I can read the incoming material from the train (and there will never be mixed materials on a single train) and set the stack size accordingly, but the stack size behaviour isn't exact - an inserter will grab only one item if only one item is available rather than waiting for a full stack. Maybe there's a mod to change this behaviour?

Edit to add screenshot and blueprint