Creationism

[GrandizerII]

For Gae, something of relevance:

Second, it may appear that Aquinas is unjustified in describing the first efficient cause as God, as least if by “God” one has in mind a person possessing the characteristics Christian theologians and philosophers attribute to him (for example, omniscience, omnipotence, omnipresence, love, goodness, and so forth.). Yet Aquinas does not attempt to show through the previous argument that the demonstrated cause has any of the qualities traditionally predicated of the divine essence. He says: “When the existence of a cause is demonstrated from an effect, this effect takes the place of the definition of the cause in proof of the cause’s existence” (ST Ia 2.2 ad 2). In other words, the term God—at least as it appears in ST Ia 2.2—refers only to that which produces the observed effect. In the case of the second way, God is synonymous with the first efficient cause; it does not denote anything of theological substance. We might think of the term “God” as a purely nominal concept Aquinas intends to investigate further (Te Velde, 2006: 44; Wippel, 2006: 46). For the study of what God is must be subsequent to demonstrating that he is. A complete account of the divine nature requires a more extensive examination, which he undertakes in the subsequent articles of ST.

https://iep.utm.edu/aq-ph-th/

So according to this analysis, God (in the Ways) should be understood to be the bare minimal, one that is exactly the "entity" concluded in each of the Ways. Aquinas isn't presupposing the triunity of God here or anything about God that requires further demonstration.

It may be that God the Trinity cannot be the same as God-as-First-Cause but that is a separate debate and has no bearing on the validity of the each of the Ways.