The OP asserts that, “Aristotle concluded that God was pure existence.” The following quote from Etienne Gilson makes it clear why this statement cannot be true. The ontology of Aristotle has distinct differences from that of St. Thomas. The former is an ontology of “essence”, while the latter is an ontology of “existence”:
“For Aristotle, it is quite true that that substance alone exist, but it is equally true that, for him, to exist is simply to be a substance, or, in other words, that to be is before all to be something. More particularly, and in the fullest sense, it is to be one of those things, which, owing to their form possess in themselves the cause of what they are. Thus, the being at which Aristotle stops is “that-which-has-the-act-of being,” minus the act of being itself. And indeed, as St. Thomas puts it, ens does not signify principally the esse, but the quod est, not so much the act-of-being as the thing possessing it: rem habentem esse. Aristotle therefore, was right in throwing into relief the role of an act played by the form in the constitution of the substance and, as well, the actuality of the substantial being, but his metaphysics has not gone beyond the plane of “entitative” being, which is that of the ens, towards attaining the very existential act of the esse.
“Here, then, is the reason for a fact noted by one of the best Aristotelian scholars, “in the verb esti, the meaning *to exist *and that belonging to the copula are strangely confused,” for “Aristotle mixes very confusedly the two senses of the verb to be, that is the being of existence and that of predication. Perhaps it would be better to say, rather than mix them, Aristotle did not distinguish them. We, who distinguish them carefully, find these two senses confused in his text. For him, to say that a just man exists, or that a man is just, was always to say that a man exists with the determination: to be just. It all came to the same thing.
“In turning the ontology and logic of Aristotle to his own account, St. Thomas transposed them from their original tone, that of essence, to his own tone, that of existence. Whence this first conclusion which is to affect our whole interpretation of Thomism: in entering the doctrine of St. Thomas, the metaphysics of Aristotle has received an entirely new existential meaning.”
(The Christian Philosophy of St. Thomas Aquinas, Ch. I: ‘Existence and Reality’)