“Christ died to take away our sins. He took away Mary’s sin ( in Catholic teaching) from the moment of her conception.”
In Catholic teaching is the ONLY place Mary was made sinless from her conception. Finally it’s in the light - the RCC made it up!
People are never sinless - we are cleansed from our sin by the blood of Christ and imbued with His righteousness. It’s Christ in the Christian that is without sin. As long as a human draws breath, he will sin. As did Mary, mother of Jesus, but NOT “Mother of God”.
Please look up Nestorian heresy. Since you agree with it perhaps you will also like Arian heresy. The two are closely related.