Modding's been a bit slowed due to IRL interruptions, but I more or less got the balance of spells the way I want them, and wrote a rough outline for exactly which megabeast/SMB should be linked to which theme, which demanded a lot of shuffling around the spheres of vanilla monsters to ensure each has a single specific spell they're associated with. If the feature I discovered in a much older test still works, they might occasionally be credited with handing out that spell during worldgen.
What's unfinished, and more relevant to Adventurecraft, is linking materials such that you can gain a spell by crafting a relevant magical essence from the critter in question. For megabeasts this'll be the heart, since crafting potions from them is already a standard convention in the mod. For bronze colossi this entails defining a new inorganic for them to use instead of bronze (I went with a material I named mountain bronze in reference to orichalcum meaning mountain copper, which only a few minor material tweaks), treants simply had the required material link added to their wood body material, while I had to retweak shoggoths to drop a gemstone-like core on death, instead of melting into a puddle of their slime (which will already be EVERYWHERE by the time the fight's over).
I'll have to make the resulting reaction very indiscriminate in what item type it looks for, because it's going to be accepting corpseparts (megabeast hearts don't butcher into meat for reaction-related reasons), logs (or potentially sticks split from it, or any random shit crafted from that), a metal statue (or anything you might make out of it if you dare to try to make after scrapping it). So I'll have to do what I did to get crafting from skulls working, make it look for REACTION_CLASS but be blind to item type, with its use of MATERIAL_REACTION_PRODUCT to avoid material loops already built in.