I'm not versed at all in McGilchrist's discussions. But ever since I first heard utterings about "Computational X" and a possible re-understanding of the notion of human free will, I've thought that something is completely missing in making the everything reducible to a primal rule.

As you pointed out in greater detail than I could, certain aspects of human behavior and being cannot be reduced to a rule that predicts all outcomes and states. Forget behavior. What are we to say of loneliness, pride, indecisiveness, magnanimity, jealousy, admiration, courage, etc.? What about the transcendental desires: truth, beauty, goodness (Plato/Aristotle), love, and home/belonging (Thomas Aquinas)?

I agree. There is much more to the human person than being governed by a computational rule.