First; you make some excellent points.
All right; you've got some salient details wrong.
The American flag did not exist until 1776, and that was only the continental colors, or 1777 for a recognizable version of the stars and stripes. And no slave ships sailed to the US after the abolition of slavery in 1865 by the thirteenth amendment - that's right, the END of the civil war, not the beginning. So the longest that "slave ships" could possibly have sailed under the "US flag" is 91 years, not "over 100 years".
Far from every slave ship sailed under the US flag, anyway. Not only did that flag not exist before 1776, but many/most slavers were from other nationalities anyway. "The Atlantic slave traders, ordered by trade volume, were: the Portuguese, the British, the French, the Spanish, and the Dutch Empire."
By far the greatest number of slaves sent to the Americas were not sent to the US or the area which would become the US, anyway. Breakdown by destination of distribution of slaves, 1519-1867:
Portuguese America, 38.5%
British America MINUS North America, 18.4%
Spanish Empire, 17.5%
French Americas, 13.6%
British North America, 6.45%
English Americas, 3.25%
Dutch West Indies, 2.0%
Danish West Indies, 1.3%
Reference: Stephen D. Behrendt, David Richardson, and David Eltis, W. E. B. Du Bois Institute for African and African-American Research, Harvard University. Based on "records for 27,233 voyages that set out to obtain slaves for the Americas". Stephen Behrendt (1999). "Transatlantic Slave Trade". Africana: The Encyclopedia of the African and African American Experience. New York: Basic Civitas Books. ISBN 0-465-00071-1.
[Note: I'm not sure what the separate categories are for, "British America MINUS North America" and "English Americas"; I don't have a copy of the reference to hand to see if/how it explains]
[BTW: re "Danish West Indies", I had to look that one up myself]
A bit of detail of which many people are unaware: the US was far from the last area to abolish slavery. Just some which held out til later were:
Portuguese territories (4 years later)
then-Spanish colony of Puerto Rico (8 years later)
then-British colony of the Gold Coast (9 years later)
Egypt (12 years later)
Ottoman Empire (17 years later)
then-French protectorate of Cambodia (19 years later)
then-Spanish colony of Cuba (21 years later)
Brazil (23 years later)
Korea (29 years later) (but not fully implemented until 65 years later)
then_French colony of Madagascar (31 years later)
then-British protectorate of Zanzibar (32 years later)
Ethiopean Empire (37-77 years later)
China (41 years later)
Siam (47 years later)
Morocco (57 years later)
Afghanistan (58 years later)
Iraq (59 years later)
Iran (63 years later)
Tibet (94 years later)
Saudi Arabia (97 years later)