You misunderstand.
Your supposition assumes time is a factor: In other words, it assumes that a choice is given and then seconds, moments, or a lifetime later a choice is made. That is not the case. That is not the way it works.
In reality, with God there is no time. Time is rather a creation of God's, a mere "image", just as we are also, just as it is written. A better understanding would be to say that time is rather a media of revelation by God, news as it were, given to the created beings within a created image. In this way, time may appear to unfold, but that is not the case. It is merely the news or revelation of what already "is", that is unfolding.
So, no, it is not that "an omniscient god negates free will", but rather that a timeless omniscient God makes freewill possible without time, as the matter is complete outside of time. To the contrary, time is contained within a timeless God. Thus, both God and those who are His are defined in the status: "I am." In this same way, it is actually correct--not that freewill comes with being born into the world, but rather "is" before the world began.
Your supposition then, is but an inside observation based on a limited logic within a close-circuit revelation of news actually existing after the fact, of which we are last to know.