So God both is morality and at the same time above morality? There is no act that God cannot do for the sake of His plan, or purpose, or what-have-you that we could ever call immoral simply because He created us. Yet He is supposed to be the yardstick that we measure our morality on. What He says morality to be is then morality, and not Him. He is not morality itself as morality that applies to us in any way has absolutely nothing to do with him. He can, at best, have knowledge of what the best things that we can do are, and what the worst things that we can do are, based on a definition of morality.
Now if we can agree on a definition of morality that doesn't demand design behind it (i.e. purpose = morality) then we can reason our way to the best possible morality without God, at least eventually. But if the definition of morality itself demands a designer, such as that definition does, then this whole argument fails because most people don't define morality as "obey God" and nothing else.
I posed the question to Sapiens as well, but I'll ask you too. If you built and programmed a robot full of emotions, and physical feelings, and aspirations, and dreams, and desires, you really feel that you have the moral authority to cause that robot pain and suffering simply because you created it?