Let’s discuss fuel, because this may be more relevant with the auto-crafter coming.
I’m team charcoal, specifically giant jungle trees. Does everything regular coal does except coal blocks, need a lot of charcoal and sticks anyway for torches and tools, giant trees consistently produce over a stack, and lumberjacking is just a relaxing way to pass the time.
The best fuel depends on where you are in the game.
Early game, before you have any farms beyond iron: anything you can burn. Logs, coal, charcoal. You’re unlikely to be doing any serious smelting jobs.
Mid game: Blaze rods (since you’ll likely have a blaze farm), possibly bamboo except you need ~82 bamboo plants per furnace to keep it permanently fueled (164 plants for smokers/blast furnaces since they burn twice as fast).
Late game: Coal blocks (since you’ll probably have a wither skeleton farm). However, that’s not automated. Once the crafter is implemented, the best fuel source will be kelp blocks - without question. You only need ~22 kelp plants per furnace to smelt without ever stopping, fully automated.