Soot sprites on the Ice levels in the mines. Even better if you can find a loot doubler ring.
Just keep swinging away. You'll get your coal. You'll get all you could ever need. The dust will never wash away. You will always see the black stains, hear the feeble cries...
Also, you can take the elevator and jump between floors 45, 55, 65 and 75 and once you cleared all the sprites go to 0 to reset the mines and repeat. Saves you huge amount of time of finding the stairs every floor.
Sure, coal was easier to find, but charcoal is renewable, and I like that.
Even ignoring that, 8 logs becomes 7 charcoal, and planting is trivial, so its just faster to farm trees in the long run. Coal is for villagers that demand it, specifically, in early trades.
Now for smelting I like bamboo, you can fully automate it. Technically you can fully automate kelp now too with the crafter, but bamboo is easy. Just break it like sugarcane and funnel it to your furnace directly, done.
If you go mining or caving frequently (and don't stay in the deepslate area the whole time) then you can build up a lot of coal, especially since with fortune III each coal block drops 2.2 coal on average
You can use stalactites and cauldrons to duplicate lava source blocks, then use lava buckets for heating the furnaces. It does take a lot of iron to make enough cauldrons though.