Mary was a sinner in need of 'God my Savior".
Mary did indeed know that God had saved her. Assuming He was required to wait until she sinned before doing that is problematic. It's also problematic to believe that a soul affected by sin could "magnify" the Lord.
Someone says to Christ "Blessed be your mother" Jesus responds "ON the Contrary - blessed are those who hear my words and do them".
Most translations are "rather", which by definition means:
1 : with better reason or more propriety : more properly <this you should pity rather than despise Shakespeare>
2: more readily or willingly : preferably <I'd rather not go> <would rather read than watch television> often used interjectionally to express affirmation
3: more correctly speaking <my father, or rather my stepfather>
4: : to the contrary : instead <was no better but rather grew worse Mark 5:26 (Revised Standard Version)>
Jesus is not denying that Mary was blessed to be his mother. To profess that puts Christ in direct contradiction with the Holy Spirit, who inspires Elizabeth to say to Mary "Blessed are you among women, and blessed is the fruit of your womb" (Luke 1:41-42). But Christ is stating that with even "better reason" she is blessed because she heard the word of God and obeyed it (Luke 1:38).
That view also puts Christ directly in contradiction with his own words to the Pharisees who felt they could set aside honoring their parents for their 'religious duty' (Mark 7:9-13). To believe that Christ would fail to honor Mary and publicly "diss" her is such a way in quite sad.
Stephen "being full of grace" is someone else who was also a sinner.
Both words rooted in 'charis', but two different forms of the word. The form used for Mary, "kecharitōmenē" is unique in all of Scripture and is in the perfect tense.
The real story here is why are they making up a story about Mary being born sinless as if they think that only a sinless person can give birth to a sinless person... when by definition they know that Mary's mother is not sinless?
I've heard some Catholics make that argument but it is indeed flawed and not the reason Mary is gifted by God with such grace.