You can't make coal out of wood, but you can make charcoal. It seems like the early recipe should be for charcoal and the later recipe (which is pretty OP for runs in which coal is scarce and needs to be manufactured) could be for coal. Minor nitpick maybe, but I would say a 1.5x boost to efficiency for charcoal over wood would be enough to make coal still more desirable if it is found, and still provide enough energy for the early game. Just my thoughts

Right, but that would involve creating what amounts to a duplicate resource to coal. Factorio isn't really that granula (see: "Heavy Oil", "Light Oil", etc.), so I figured just reusing coal would be fine. You are correct that charcoal should have a lower energy density, but I'm handling that via the recipe's ratios. The Charcoal recipe gives you 1.5x the energy value of the wood you burned to make it, plus the energy it cost to make it (to run the furnace, relative to an Electric Furnace). You can alter this ratio in Mod Settings, and the mod will recalculate the proper wood-to-coal ratios for it.

Mind you, I'm not intrinsically opposed to creating obscure resources (else Planetary Fuels wouldn't exist), but creating a duplicate to coal seems like a lot of work for not much benefit. I'd have to consider what recipes it could be involved in (like making grenades) and what the energy cost would be, mess with graphics for it, and all for 0.75 coal. I think I'm willing to accept a bit of jank in his case :)