An update after getting my system running, one important thing to know is apparently trains need some default schedule for interrupts to function. I spent at least an hour trying to figure out what was wrong with my circuit based interrupt until I tried putting a station destination into its schedule and it just started working. Now I just need to replace all my stations.

I'm now decoupling my bus from the furnaces and making them train based. I'm excited to see it all working and being demand-based.

I'm now thinking about making my furnaces material agnostic and the outputs just get sorted by type. Maybe that's a project better left for later so I can actually play the DLC...

Edit: I realized why it needed a station in the schedule. It can't read the circuit network unless it's actively stationed at the station. If it's just sitting there but not actively set to be there, it doesn't receive the signal. That's reasonable I guess, but good to know.