How can I will a maxim to become a universal law, surely that kind of power can only be wielded by a god?
The categorical imperative was to Kant a universal law, I think it can be useful to think of it as an example when considering the positive features these laws would possess. I think your confusion may be based on the difference between laws which you would be happy to see regulating the conduct of all humans, and natural laws governing the movement of celestial bodies etc. A god/God would be a likely candidate for creating the latter type of laws, however Kant did not seek to evince his arguments through a transcendent guarantor of truth, as for example Descartes does in the Meditations. The categorical imperative is about reasoning for oneself about whether the maxims informing what you do/don't do are ones which you would also will to become a law of conduct which all people should act in accordance with, not deciding for oneself the essential nature of humans. Universalised maxims relating to conduct, even though they are framed in terms of people being happy to will them to become universal laws, are not the same as natural laws.