Part of the problem with these arguments for “God”, I think, is that they don’t go deep enough into what the divine attributes mean, such as simplicity or transcendence. Let’s take these for illustration:
Divine Simplicity:
To be divinely simple requires that one has no parts of any sort – physical, metaphysical, temporal, etc. This is all well and good to acknowledge, and some even go on to recognize that in having no parts of any sort, one must – in a radical sense – have no “nature”, “species” or “form of being.” But, it is never realized that this entails that one who is divinely simple cannot be the only one who is divinely simple: such a statement makes no sense, as “divine simplicity” cannot be a ‘property’ or ‘feature’ at all, let alone one which only one has.
There can only be one without contradicting the Law of Identity. I’m not sure where you get that "it is never realized that this entails that one who is divinely simple cannot be the
only one who is divinely simple. This has certainly been framed in more contemporary terms, but Aquinas was sure to make the point in his work Summa Contra Gentiles.
Book 1 - Chapter 42:
[3] Again, it has been shown that God is absolutely perfect, lacking no perfection. If, then, there are many gods, there must be many such perfect beings. But this is impossible. For, if none of these perfect beings lacks some perfection, and does not have any admixture of imperfection, which is demanded for an absolutely perfect being, nothing will be given in which to distinguish the perfect beings from one another. It is impossible, therefore, that there be many gods.
[8] Then, too, if there are two beings of which both are necessary beings, they must agree in the notion of the necessity of being. Hence, they must be distinguished by something added either to one of them only, or to both. This means that one or both of them must be composite. Now, as we have shown, no composite being is through itself a necessary being. It is impossible therefore that there be many beings of which each is a necessary being. Hence, neither can there be many gods.
[9] Furthermore, given two gods that are posited as agreeing in the necessity of being, either that in which they differ is in some way required for the completion of their necessity of being, or it is not. If it is not, then it is something accidental, because that which accrues to a thing without contributing to its being is an accident. Hence, this accident has a cause, which is, consequently, either the essence of the necessary being or something else. If its essence, then, since the necessity itself of being is its essence, as is evident from what was said above, the necessity of being will be the cause of that accident. But the necessity of being is found in both gods. Therefore, both will have that accident, and thus will not be distinguished with reference to it. If, however, the cause of the accident is something else, it follows that, unless that something else existed, this accident would not exist; and unless this accident existed, the aforesaid distinction would not exist. Therefore, unless that something else existed, these two supposed necessary beings would not be two but one. Therefore, the proper being of each depends on the other, and thus neither of them is through itself a necessary being.
[15] Furthermore, either the nature signified by the name God is individuated through itself in this God, or it is individuated through something else. If through something else, composition must result. If through itself, then it cannot possibly belong to another, since the principle of individuation cannot be common to several, It is impossible, therefore, that there be several gods.
[16] If, again, there are several gods, the nature of the godhead cannot be numerically one in two of them. There must, therefore, be something distinguishing the divine nature in this and in that god. But this is impossible, because, as we have shown above, the divine nature receives the addition neither of essential differences nor of accidents. Nor yet is the divine nature the form of any matter, to be capable of being divided according to the division of matter. It is impossible, therefore, that there be two gods.
And there’s 24 paragraphs in this chapter, and this chapter is built on the 41 chapters before it, and this is only intended as a helpful handbook to already educated priests in non-Christian areas, but suffice it to say, much mind has been paid to the topic by both Aquinas and all other proponents of divine simplicity.
To be transcendent is to be prior to every category of being – object, property, event, state of affairs, etc. This is all well and good to acknowledge, and some even go on to recognize that in transcending every category of being, one must – in a radical sense – be unclassifiable. But, it is never realized that this entails that one who is transcendent cannot be the only one who is transcendent: such a statement makes no sense, as “transcendent” cannot be a ‘category’ at all, let alone one that has only one member or instance.
Again, it follows for the same reasons detailed above. “Transcendent” isn’t some arbitrary title or property. We’re not just throwing the word around. We’re referring to an act of existence which has no limit or boundary, which remains unconditioned, of which it is pure act and no potential. It might due to read up on some Thomist materials regarding what is meant by “act of existence” and “essence.”