So after dropping the millionth stack of human flesh and 3 whole arms while, ahem, fulfilling some corpses with Derketo’s kiss, I thought “enough is enough!” and came to the place everyone goes with their gripes:
My very simple suggestion is this: religion weapons only harvest religion materials. No increased drops, just a specific tool for obtaining specific materials. You can harvest human flesh with plenty of other weapons, but if a Set acolyte is carving out a person’s heart, it’s unlikely they’re going to take their legs as well. Unless that’s their thing, in which case there’s plenty of legs in the Exiled Lands and axes to chop them with.