OK... you can do this like you said by putting ores in each slot.. which is the thing that is slow... like a very boring thing to do 1000s of times.
Sometimes it misses a slot.
It's just really finnicky to do this over and over again and its boring. If there is a way to write a plugin to do that well great but I don't know anything about plugins for this game.
I read in the steam forums at some point over the years of playing. That someone asked if we could click and drag so that they would fill up faster. I believe that Red said that he would add it to his (probably very long) list. There could potentially be an item that you have to craft to make this happen. One other suggestion I would like to see is a backpack. My inventory fills up quick.
If he said its on the list, great. Fine.
To be honest that's part of the game! Maybe someone could write a plugin to do this? But imo manual loading of furnaces is part of the survival experience, if you don't want that maybe try playing in creative mode? No need to fill furnaces then.
I know its just part of the time game right now but I'm really sure thats a big turn-off for many people. We can put all those people who want to bypass that in the lazy/cheater category but man, Its just the repetetiveness t hat kills.
You could say the same thing about mining....bang bang bang on the ground over and over again BUT BUT..... the ground breaks in unique ways and you never know for sure what's underneath your digging...and you're also collecting materials which again is toward your future goals.
It's just different man. *runs away crying*