Really, though… no, not in a chronological sense. We cannot say “at this point in time, only God existed, but then at this later point, both God and creation existed.”
God experiences all things simultaneously and eternally. However, he created the universe (which means that he created time itself), and one of the features of his creation is that it has a temporal dimension.
God created the universe. Another way of saying this is to say that his existence is ‘prior’ to the universe. This doesn’t mean that his existence is chronologically before that of the universe (since time is a feature of the universe itself), but only that he created the universe (and therefore, the universe does not precede him or is created along with him).
No. God is not bound by time. He created it.
So, if we disagree with your assertion that “God is subject to time”, then we don’t have a problem with him being the ‘uncaused cause’? OK… I’m good with that: God isn’t subject to time, and is the sole uncaused cause. Cool.