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 out what the conventions are for this field, thus far unsuccessfully)

Article 5: I still don't understand properly, but do see that Aquinas' conclusion is that our creation is not the only possible one (and is therefore contingent?) See note about maximum vs. maximal below.

Article 6: IIUC: things consist of their essence and their accidents. If two things share the same essence, we say one is greater than the other if its accidents be greater. It is possible to "max out" the accidents, however, and in that case to create something "greater" one must create something with an essence which is better than the original essence (as 5 is greater than 4; natural numbers having no accidents).

[I fail to find Aquinas' reply to objection 4 convincing, or even necessarily relevant? As far as I do follow it, it does imply that "greater" is only a partial order, as Mary and created happiness (whatever that may be) are both coatoms in this order: anything which is at least as great as Mary must be either Mary or God, and similarly for created happiness] see https://en.wikipedia.org/wiki/Atom_(order_theory)#Coatoms

As the reply to object 3 shows, this creation may be the greatest possible creation of its essence, but even if God is limited to creating only the "best" creations, he can always create a universe whose essence is greater than that of ours. (here I am imagining that the essence of a universe might be the set of the possible essences within that universe, and the accidents of the universe would be the set of accidents of incidents of those essences, and the set of all the relations between them) So for instance, our universe is the best possible universe with only 7 colours, but God could create a universe with an 8th colour ("octarine" without loss of generality), and then he would have to create a "best possible" universe under those constraints (todo: the maximum universe, or merely a maximal universe?), leaving open the choice of creating universes yet greater.

For example (taking a small model because we lack infinite wisdom), we could say that possible universes are grammatical english sentences, the essence of a universe being the words contained in it, and the accidents being the position of those words within the sentence. Furthermore, sentence A is greater than sentence B if it can be formed by adding words to B, without rearranging B. Examples: "See Spot run" is greater than "See Spot"; "The man upon the stair wasn't there" is greater than "The man wasn't there"; "See Spot run" and "Dick and Jane" are unrelated: neither is greater than the other. Essence A is greater than essence B if all the potential words of B are also potential words of A, so "See Spot run quickly" is greater than "Quickly, see!" even though there is a rearrangement, because its essence is greater.

From this model we recover Aquinas. Given many essences, it is possible to create the greatest sentence using only those words; {"Run","See","Spot"} has the sole possibility of "See Spot run"; {"fast","he","is","very"} has no greatest sentence, for one can always add another "very"; and {"Dick","Jane","kisses"} has two greatest sentences, for "Dick kisses Jane" and "Jane kisses Dick" are incomparable, however, there are greater creations, for {"Dick","Jane","kisses","passionately"} is a greater essence (with 6 maximal creations).

What errors have I made thus far?