So Robert, do I understand you correctly that as an agnostic you are not sure what to believe about God except that you are sure Christianity is wrong?
God didn’t choose Mary as arbitrarily as you seem to imply. From the very beginning, God knew we would sin & fail, so from the very beginning He knew He would send His son to redeem us. He knew that would be through Mary long before she was born. It was all part of His plan.
I agree that you do not understand His plan, and frankly, neither do I; no one can completely grasp the plan of god. If, with our puny human intellect, we could understand the plans of God, He wouldn’t be a very impressive god, would He?