God is subsistent act of existence itself, yes.
But every other reality, insofar as its a unity in itself — a “being” — is existence limited by an essence. Such that, existence is the maximum (and not a minimum), and essence is not a positive, but more like a negative or limitation of being.
You see that, on this view, individuals beings would not make up being itself (God), because being itself is fully actual, infinite, unconditioned, etc. This Unconditioned Reality is distinct from other acts of being, because all other acts of being are limited versions of being — while, again, the Unconditioned Reality remains existence in its fullness.
In a book called The One and the Many, the author (who is a Thomist) recommends we think of an individual reality like a cat as “an act of existence acting in the manner of catness” (paraphrase) as an exercise in Thomistic metaphysical thinking.