Xians often say God is a Person. This is not so much a rational proposition as a ritually devotional utterance.No, He's a self-existent Person. He has His own epistemology, and His own ontology. No part of Him depends on man, or on what man knows or thinks. God pre-existed man, by definition, since He created man. And there was an infinite amount of time (if we can even use that word of eternity) when no men or women existed. That much we can be sure of, since you and I, and all mankind, are contingent beings of some eighty years or so duration, at best.
Pantheism does have he problem you cite, though. Since it imagines God is Monistic, it has to posit that creation is necessary and eternal (two things we can easily observe it's actually not), and that its god needs there to be a physical world in order for the god itself to exist. The Pantheistic god is not self-existent but dependent...hence its unavoidable appeal to Material-Spiritual Dualism, with both being eternal.
But that's only another reason why Pantheism is wrong. It's not even plausible scientifically, since the material world is obviously contingent and temporal, not eternal.
God does not "pre-exist" anything, because 'pre' refers to temporal dimension. The eternal aspect of God rules out limitation to time and any other relative dimension.
God is temporal only in His temporal aspect but He is not temporal in His eternal aspect. For all we can know, besides the temporal and the eternal there could be infinitely other aspects of God we can't even guess at.
I did not say creation is eternal I said it is a fact (it is indubitable that something is happening). Eternity, the eternal now, is uncreated as far as time and other dimensions are concerned. Time and all the other dimensions are man made concepts.
By "pre-exist" I think you may have intended ontic precursor. If so, I agree but as faith not as reason. Reason does not extend into that region.