How did declaring that one particular ethnicity of human to be unsuitable for emancipation and citizenship advance the goal of “a more perfect union”?
You miss the point. What guarantees our Free Constitutional Republic is the rule of law of the Constitution. The rule of man and his personal whims is the basis of tyranny.
The seeds of the Civil War were sown in the slave trade of the South and the need for the southern states to ratify the Constitution to create the United States of America. The compromise to not forbid slavery in the Constitution was absolutely necessary to form a more perfect union by establishing the Constitution. The Constitution formed a more perfect union, not a perfect union.
Nothing this side of heaven is perfect but we strive as best we can. The Lying Left gets its ammunition for more and more government tyranny by focusing on the small % of freedom that is imperfect. But the Constitutional basis of our Free Constitutional Republic was and is way ahead of whatever is in second place.
Don’t just point out a flaw. Have ready an alternative that improves freedom and happiness.
The Dred Scott case was not about any "ethnicity" en masse, but about a particular individual who was legally a slave at point A, but (perhaps) not at point B. Every state had free people of African descent.