This goes much further than a rejection of Divine Simplicity for Mormons, it seems to me.
You keep repeating that Latter-day Saints worship a "different God," but that conclusion simply assumes that your Thomistic definition of God is correct. You haven't demonstrated it.
Compare these two...
The God of Abraham speaks. He acts. He loves. He grieves. He enters covenants. He responds to prayer.
Your God is an absolutely simple, timeless, immutable essence whose knowledge, power, justice, mercy, and love are all literally identical. In Thomism, God doesn't merely have attributes - God IS those attributes. Justice is mercy. Mercy is power. Power is knowledge. At some point you've philosophized the God of the Bible into an abstraction.
Then you claim Mormonism cannot explain existence because eternal matter cannot explain itself.
Why not?
You allow God to be self-existent. You allow God to be a brute fact. You allow God to require no further explanation.
But the moment a Latter-day Saint says some aspects of reality are eternal, suddenly you're demanding an explanation beyond the ultimate explanation.😂
The truth is that your worldview also bottoms out in something unexplained. You stop with "Pure Act" or "Being Itself." We stop with an eternal reality inhabited and governed by God. Neither view escapes having an ultimate stopping point.
And your appeal to "God as the very act of being" doesn't actually explain why anything exists. It simply labels the mystery.
Saying "God is Existence Itself" sounds profound until someone asks the obvious question: Why does Existence Itself exist?
At that point, the Thomist does exactly what everyone else does - he declares his preferred stopping point necessary and refuses to go further.
MOST IMPORTANTLY...
You haven't shown that creation ex nihilo is biblical. Genesis does not teach it. The Hebrew text does not require it. The earliest biblical authors never stop to explain this supposedly foundational doctrine. Instead, what we repeatedly see is God bringing order from chaos, not absolute nothingness.
So the argument ultimately reduces to this...
"Thomas Aquinas defined God one way. Mormonism defines God differently. Therefore Mormonism is false."
😂
That is not an argument. It is a declaration of loyalty to a particular metaphysical tradition.
You are free to prefer Thomism. What you have not done is prove that the God of Thomism is the God of the Bible, that creation ex nihilo is required by reason, or that an eternal reality is somehow less intelligible than a timeless, absolutely simple being whose essence is allegedly identical to existence itself.