You are very correct that the text of Luke 1:28 does not necessitate the Catholic doctrine on Mary; it witnesses to it. This is the case with all Catholic doctrine: the Church discerns the doctrine from its sacred tradition by looking at available historical knowledge of the life of the Church. The Church does not do what the Protestants do, read the scripture and in it find the doctrine. Rather, both the Holy Scripture and the doctrine are interrelated products of the Church.
Indeed one only reading Luke 1:28 may build theories that (1) Mary reached age of reason (one cannot sin before the intellect is mature enough to recognize sin), committed a unknown to us and unrecorded by St. Luke sin, then received the fullness of grace from Archangel Gabriel, and since then remained sinless thanks to that grace; or (2) Mary was sinless like any child, received the fullness of grace at some point, and in her old age the grace aired off and she committed an unrecorded unknown to us sin.
The reading that since the fullness of grace preceded the arrival of the angel it must have been there from her conception; and that grace does not go stale once received, and that God has the power to raise any kind of Mary He wants for His own mother, and probably would not want a sinner in that role -- that reading, the Catholic one, or rather that doctrine, had existed inside the Church even before Luke, under the dictation of the Holy Ghost, wrote his gospel.
Why would the "Catholic one" suppose that God "probably would not want a sinner in that role (the mother of the incarnate Son of God)"? Was not one of the main intents of the incarnation that Jesus was:
For what the Law could not do, weak as it was through the flesh, God did: sending His own Son in the likeness of sinful flesh and as an offering for sin, He condemned sin in the flesh, so that the requirement of the Law might be fulfilled in us, who do not walk according to the flesh but according to the Spirit. (Romans 8:3-4)
And:
Since the children have flesh and blood, he too shared in their humanity so that by his death he might break the power of him who holds the power of death—that is, the devil— and free those who all their lives were held in slavery by their fear of death. For surely it is not angels he helps, but Abraham’s descendants. For this reason he had to be made like them, fully human in every way, in order that he might become a merciful and faithful high priest in service to God, and that he might make atonement for the sins of the people. Because he himself suffered when he was tempted, he is able to help those who are being tempted. (Hebrews 2:14-18)
There WAS no need for a "sinless" mother in order for the Christ to be born and be without sin so that He can make atonement for the sins of the world. That Jesus is God incarnate is how he could be sinless - it had nothing to do with His mother also needing to be. Mary was a human woman, born under the law, born under sin, and her faith in the Savior Jesus Christ is what saved her and cleansed her from all her sins just as He does for us.
That really IS the problem, Catholicism decides what is doctrine whether or not God's sacred word states it is so. I believe they have it exactly backwards. God tells us what is truth and our response is to believe it and obey. We don't get to make it up as we go along and superficially search for verification from Scripture afterward.
Yeah, those stinking sinners. Who needs them? They're just not good enough for God. Or for God to even use.
Romans 5:6-8 For while we were still weak, at the right time Christ died for the ungodly. For one will scarcely die for a righteous person—though perhaps for a good person one would dare even to die—but God shows his love for us in that while we were still sinners, Christ died for us.
Another thing to note.... If you think that Mary was sinless and yet still full of grace, it appears that you do not have the slightest clue what grace is all about.
One cannot be graced if one is without sin. Grace is ONLY for sinners.
So Mary cannot be sinless and yet graced. The whole premise of Mary being sinless falls apart.
Which is selectively done, as such things as the IC lack unanimous consent of the fathers, and the Assumption lacks weight of early testimony , and in other things Rome differs with the EOs on what Tradition teaches, even as in the case of universal papal jurisdiction and papal infallibility, among other things.
The reading that since the fullness of grace preceded the arrival of the angel...
That this grace was perpetual sinlessness is presumed, versus that grace being that this evidently holy Spirit-filled soul had "found grace with God" as the text states, (Lk. 1:30) in being chosen to be the God-bearer.
God has the power to raise any kind of Mary He wants for His own mother, and probably would not want a sinner in that role
One could argue that God would not want the instruments out of whom flowed God-breathed Scripture to be sinners either, or Mary's parents, etc.
But doctrine is not based on what God may or could have done, or silence, but what the evidence testifies He did do. Lacking that, all that is warranted is speculation. Notable exceptions to the norm (excess fingers, long age, celibacy, absence of parents, sinlessness, etc) are characteristically provided in Scripture, even of lesser characters, and the sinlessness of Christ is stated at least thrice. Marian lifelong sinlessness and virginity are not, but are read into the text based upon the fallacious idea of necessity, or possibility.
In contrast to all that attributed to her, the mention of Mary is relatively marginal in Scripture, while the Holy Spirit gives far more description of Paul's sacrificial labor of love in birthing spiritual children and churches, and working toward their maturity. And the Holy Spirit never manifestly attributes sin to him after his conversion, but to say that he was sinless would be presumptuous.
The Church does not do what the Protestants do, read the scripture and in it find the doctrine.
That is rare admission, as too many RCs try to argue as if they were evangelicals in trying to wrest support for Rome's traditions as if their veracity rested upon the weight of Scriptural warrant.
But by making her amorphous oral tradition equal with Scripture, selectively channeling ancient stories into binding doctrines, then Catholicism (sometimes in conflict), has perpetuated mere traditions that developed over time.
The oral "traditions" Paul wrote of were not that of ancient tales, but known contemporary preaching which we rightfully can expect were subsequently written, as is seen elsewhere with revelation called "the word of God/the Lord."
Details hitherto unknown as recorded could be revealed (cf. 2Tim. 3:8) but which we know by Scripture, which is the assured word of God and supreme standard for testing and establishing Truth claims. Making the church autocratically supreme is contrary to its basis of establishment, that of Scriptural substantiation in word and in power, and making her oral tradition as equal with Scripture is essentially adding to the canon.