Definitely helps to know my hunch about the .dll thing was correct, and the additional information is much appreciated. Thank you.
I know there's some new content in 1.6 that kind of adds a vanilla industrial furnace (the sprite is the same size as the old furnace though, so couldn't just do a recolor with seasonal skins. But if you're worried about how good of a coding example that Furnificent thing would be, the base game code might have answers now.
https://stardewvalleywiki.com/Heavy_Furnace
The Heavy Furnace can do 25 ores at once using 3 coal. I'm not sure if it's easy or difficult to increase the amounts of those, but it's something to consider. I feel like the amounts and times it takes to process should all be editable. (But I say this as someone who hasn't looked at the base game code).