Original Sin isn’t spelled out utterly and completely clearly in Scripture, and Scripture certainly doesn’t have a passage stating “Mary was conceived with original sin”.
That said, St. Thomas provisionally agreed with you, although St. Augustine provisionally made no comment, and Bl. Duns Scotus, of all people, ended up explaining how it all worked.
I guess all have sinned excludes Mary??
Lucy, this ‘splained it purty well to me...
Romans 5:12-21
12 Therefore, just as sin came into the world through one man, and death through sin, and so death spread to all men[a] because all sinned— 13 for sin indeed was in the world before the law was given, but sin is not counted where there is no law. 14 Yet death reigned from Adam to Moses, even over those whose sinning was not like the transgression of Adam, who was a type of the one who was to come.
15 But the free gift is not like the trespass. For if many died through one man’s trespass, much more have the grace of God and the free gift by the grace of that one man Jesus Christ abounded for many. 16 And the free gift is not like the result of that one man’s sin. For the judgment following one trespass brought condemnation, but the free gift following many trespasses brought justification. 17 For if, because of one man’s trespass, death reigned through that one man, much more will those who receive the abundance of grace and the free gift of righteousness reign in life through the one man Jesus Christ.
18 Therefore, as one trespass[b] led to condemnation for all men, so one act of righteousness[c] leads to justification and life for all men. 19 For as by the one man’s disobedience the many were made sinners, so by the one man’s obedience the many will be made righteous. 20 Now the law came in to increase the trespass, but where sin increased, grace abounded all the more, 21 so that, as sin reigned in death, grace also might reign through righteousness leading to eternal life through Jesus Christ our Lord.
My soul magnifies the Lord, and my spirit rejoices in God my Savior, for he has looked on the humble estate of his servant. Luke 1:46-48
For there is no distinction: for all have sinned and fall short of the glory of God, and are justified by his grace as a gift, through the redemption that is in Christ Jesus, whom God put forward as a propitiation by his blood, to be received by faith. Romans 3:22-25
If we claim to be without sin, we deceive ourselves and the truth is not in us. If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness. If we claim we have not sinned, we make him out to be a liar and his word is not in us. (1 John 1:8–10)
(Is this true? If you claim that Mary is without sin then you are saying God is a liar.)
You write this:
Scripture certainly doesn’t have a passage stating “Mary was conceived with original sin”.
But God's Word DOES say that "all have sinned" and Mary, blessed as she is, is included in that crowd.
Using your logic, you would need to find a "passage" that demonstrates that Mary was sinless. You can't do that - Luke 1:42 doesn't meet your need. Neither would any attempt to say that she was "full of grace" (Luke 1:28) and therefore sinless. That's a fail, too - unless Stephen was also sinless by the same description (Acts 6:8).
Finally, I won't be replying to you (unless you change your POV) because I've been in this rodeo a long time and even though the clowns are new - the act is the same.