I'll just baldly give the theory without much argument. Axiology is necessary. It's a necessary truth that friendship and knowledge are good, that false beliefs are bad, etc. But the values have many aspects and exhibit much incommensurability. It's also a necessary truth that the good us to be pursued and the bad avoided. This gives some practical guidance, but mainly in cases where the reasons in favor of an action dominate those against. And that's rare. In typical cases agents face competing incommensurable reasons.
There may also be a necessary truth that some goods are fundamental and never to be acted against. The nature of a particular kind of agent then specifies how incommensurability is to be resolved. When the agent should be merciful rather than strictly just, when strictly just, and when the agent is morally free to go either way. The nature of an agent also gives the agent inclinations to act accordingly, inclinations that can be introspective. So we can know how we should resolve cases of incommensurability when they come up for us. We have reliable moral intuitions about us.
But these moral intuitions are about humans. Intelligent sharks would have a nature that resolves incommensurables differently, and our moral intuitions wouldn't directly tell us much about how intelligent sharks should act (except in cases of domination and maybe the deontic constraint not to act directly against the most fundamental goods). So we have a reasonable scepticism about for our insight into how a morally upright intelligent shark would act. But this scepticism of course in no way detracts from our knowledge of how we should resolve incommensurables.
For exactly the same reason, we have a reasonable scepticism about how God would act, about what resolutions between incommensurables are necessitated by his nature and which are left to choice. But this scepticism in no way detracts from our moral knowledge.
I don't think the scepticism is total. We can engage in limited analogical speculation. But this needs modesty if the theory is right.
Let me end with a little argument. When we think of particularly outlandish ethics cases, such as actions that affect an infinite number of people, we get stuck or even misled. No surprise on the above theory. We aren't made for such decisions. Those are decisions for more godlike beings than us. Perhaps our nature simply fails to specify the resolutions for these cases, as they aren't relevant to us in our niche. Imagine asking an intelligent amoeba about sexual ethics!