I recently unlocked Growling Grotto, and noticed that more than ever I'm ending up with tons of remnants when smelting. I think it would be more appropriate if each mining area only had the top 3-4 types of ore available at that point. So by Growling Grotto, copper should no longer drop, Iron no longer drop in Fluorescent Fountain, and Steel no longer drop in The Deep Deep. By that point, the lower ores are basically useless, and even if a user for some reason wants a lower tier of metal, they can just go back to earlier zones for it.
As it stands now though, it instead takes either micromanaging what ores are in your cart before you go to smelt, or coming back with half a cart of unsmelted ore because of how many different types there are by the later areas, and that's just frustrating design.