In Educated: A Memoir by Tara Westover, what is Westover's central argument, focus, or main idea?

This is by no means a definitive answer, but I would argue that the central idea of Educated is the power of the human spirit to overcome the greatest adversity.
Just look at all the struggles that Tara has to endure from an early age. Regularly forced to travel from place to place by her self-centered, eccentric parents, Tara is never able to experience the kind of settled, stable lifestyle that is absolutely crucial to a child's emotional and educational development. Mired in poverty and not receiving an education, it would seem that Tara is destined to spend the rest of her life on the very bottom rung of society's ladder.
Despite all this, Tara has something deep inside her which allows her to transcend the manifest limitations of her chaotic, unconventional upbringing. Call it the human spirit, if you will, but whatever it is, it never dies, despite the best efforts of Tara's parents to kill it. Come what may, Tara is determined not to be like them; she's firmly committed to going out into the world to make a name for herself; to be educated, financially comfortable, and emotionally secure. That she is able to achieve all these goals bears eloquent testimony to the remarkable strength of spirit that burns deep within her soul.