Earlier quoted context omitted.
For the moment, let's put any "part of/containment/belonging to" relation between creator and created aside (in part due to the subtleties you note; in part because I'm more interested in the necessity and possibility of creations, and —despite having slept on it— don't understand Aquinas yet). (in parentheses: sorry for my ad-hoc notation earlier; I've been looking for a survey paper on Algebraic Theology to find ou…
Sorry for the delay. I think what you've said is generally correct in that: yes, Aquinas thinks that making a thing better is possible insofar as its accidents can be made better, but not its essence since changing its essence would make that thing cease to be that thing. Your sentence analogy is apt. Aquinas does indeed think creation is contingent (though someone like Leibniz does too---it's just that Leibniz think…
Aquinas' point is highly relevant: if both God and his creation were infinitely perfect, they'd be indistinguishable and we'd be back at pantheism.