‘from the end of the American Revolution (1783) until the Emancipation Proclamation (1863)’
slavery was freely practiced throughout the eighteenth century in the colonies, mainly on the rice, indigo, and tobacco plantations along the southeastern coast; should you not add that period to your 80 year reckoning...?
Key phrase: “this country”
What happened before the independence of this country is not our so-called “original sin” - it was determined by European monarchs an ocean away. Please don’t underestimate the importance of that point.