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 thinks God is obligated to actualize the best possible worlds as opposed to other possible worlds, but such worlds are still possible, and thus contingent.) The idea that creation is necessary is somewhat fringe (among theists and atheists alike). The idea that all things that are possible are necessary, modal collapse, is generally seen to be so untenable that if a consequence of an argument is modal collapse, it's almost seen as a reductio ad absurdum. This is the case for, for example, Gödel's ontological proof of God.
Maybe one relevant item here is that Aquinas thinks God is the most perfect, and that God cannot make creation to be as good as him---so while it might be true that God can make something that is better than a particular thing (by making something with a different essence), he cannot make something infinitely perfect (the only such thing is himself).
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 thinks God is obligated to actualize the best possible worlds as opposed to other possible worlds, but such worlds are still possible, and thus contingent.) The idea that creation is necessary is somewhat fringe (among theists and atheists alike). The idea that all things that are possible are necessary, modal collapse, is generally seen to be so untenable that if a consequence of an argument is modal collapse, it's almost seen as a reductio ad absurdum. This is the case for, for example, Gödel's ontological proof of God.
Maybe one relevant item here is that Aquinas thinks God is the most perfect, and that God cannot make creation to be as good as him---so while it might be true that God can make something that is better than a particular thing (by making something with a different essence), he cannot make something infinitely perfect (the only such thing is himself).