OK, it has taken me long enough, but after revisiting the first nine steps of my 10-step argument for God, I think I have a tenth step. And it's very short and simple.
Generation or constitution? The only alternative can be generation, i.e., creation ex nihilo. Why? Because constitution is only possible within limited beings. Consider a sheet of thin white paper with red marker scribbling on it. The paper is the substratum of the difference between the white and the red on the paper. Because the dye of the marker has soaked the paper through, the paper molecules can be said to be constitutive of the difference in colors. But the reason why it is so is because the paper itself has ceased to be uniform.
H2O is constitutive of the difference between wet and hard inside a glass of water; but this is so only because of the non-uniform alignment of the molecules within space. Anything that is non-uniform cannot be God because interior limits imply a further substratum; i.e., in the case of the colored paper and the ice water, the substratum is space.
At the final border between limited being and nothing at all, there can only be one ultimate substratum. For being to be limited, thus substratum must have a will. And in order for this substratum to be absolutely ultimate, it's relationship to limited being must be creative rather than constitutive.