> The claim about the validity of Aquinas' ideas seem to only make sense if we accept his initial assumption which is the existence of God.
Not at all. Aquinas does not begin with the existence of God. Rather, he shows how the existence of God can be inferred from basic metaphysical principles. So you have things exactly backwards.
It is a common misconception that the existence of God must be assumed or that it is a matter of faith.
> The confusion here is that there is no such thing as "universal good". It is always "good for something" and "good for somebody". Bad for something else, and bad for somebody else. So the claim seems to be that because something is good for somebody and bad for somebody else, there is perfection?
No. What determines the good is the nature of a thing and its telos. To be a good human being is to more fully actualize human nature and its end. What it means to be a good turkey is to more fully actualize turkey nature and its end. Perfection is the case when a thing fully actualizes all its potential as the kind of thing it is; there is nothing left to actualize. (Accidentally, turkeys can exist for the sake of the good of others animals, but this is secondary to what is intrinsically good w.r.t. a kind of thing.)
In the case of God, the nature of God is to exist, and God is goodness itself (a transcendental).
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I encourage you not to be flippant about metaphysics. It deserves and requires no less attention and seriousness than any other science, and indeed more, as metaphysical principles by definition underpin all of reality. Get them wrong, and things go wrong.