This is an excellent question. There are two things I think you should read over and ponder very carefully: Summa I I Questions 44 - 49 and St. Thomas on The Eternity of the World
dhspriory.org/thomas/DeEternitateMundi.htm
The answer to at what “time” did God create the beginning of the universe is a bit misleading. You are trying to imagine a temporal “before” before time itself, which is impossible. You are still thinking of creation as a temporal cause which gives rise to a temporal effect.
Imagine, however, a footprint in the sand. Do not think of how it currently got there, but only that it is, presently, there in the sand. The causal relationship that exists
now between the footprint and the sand is a simultaneous one,
not a temporal one. The sand is simultaneously sinking because the foot is simultaneously causing it to sink. We see here in this example that there is a certain hierarchy of causality which is independent of time. Another example: suppose you are sitting in a chair. It is true that the relationship between you and the chair is simultaneous, yet there is a causal order. You are sitting because the chair is holding you up. Without the chair, you could not be sitting, though the chair could still be there. The chair, in this example,
causes your sitting in a certain way, since, if it were lacking, it would not be possible for you to be sitting in a chair.
Now, the creation of the first moment of time is the same. There is no temporal moment before it. Rather, the effect of time is *simultaneous with *the cause of God giving rise to it. It is true, there is no temporal moment in which time does not exist. Yet there is a “logical” or “causal” moment, since their is an “order of nature” (as some philosophers say) in the relationship between God causing time and time existing.
Hope this helps.