would be a timeless entity
More precisely: eternal.
What that entails is that all of that "first cause" happened in an instant that took no time.
This premise neglects the definition of eternal as outside time. In this definition of eternal, "instant," "time," "no time," "some time," etc. are nonsensical.
Eternal means forever, and again implies time. In your hypothetical realm that was created to accommodate the deity which has no time to speak of, eternity is equally meaningless - no difference between a nanosecond and a trillion years.
As 'time' itself wouldn't apply, change cannot happen without time, and therefore, nothing that the deity did would / should take any time, implying that everything that it did, it did in an instant, and not over a course of days: "On the first day... rested (a time-based non-action) on the seventh day," etc.