The implication seems to be that she was holding back quite a bit on Nathanos, considering how she basically just blinked his dogs out of existence. I doubt that would have been possible before. It was in her interest to at least see if he would give up any useful information.
Did anyone read the Mistborn series? It takes time to get used to deification, to be able to use such powers. Think of it like becoming Preservation. It will come slowly but surely. There’s no need to panic about her not being portrayed as too strong yet, it will come. The slow burn of its growth gives it more gravity.