You're absolutely right, the scale of the transatlantic slave trade was greater than ever before seen in human history.

Somewhere between 1 in 5 to 1 in 6 died on the journey alone.

One needs to also be aware that slavery as practiced by modern Western colonial empires was even more cruel than how slavery was practiced traditionally even dating back to antiquity.

In most of the world slavery was often contingent and there were several pathways to manumission (freedom). It was not an inherited status by default (partus sequiter ventrum).

The Western colonial empires very much perceived the entire world according to a race based caste system where Europeans were the highest caste and those of African descent were the lowest. Complexion defined your worth as a person. They spread this worldview globally, which has poisoned so many minds over centuries, and the world is still recovering from it to this day.

America adopted and upheld this race based caste system until the civil rights act passed in the 1960s (within the lifetimes of many of parents/grandparents). During the American era of slavery, slaves had no legal rights. They did not have personhood. Children of slaves were automatically also slaves from birth. Slaves could be grieviously injured or killed with impunity. They were seen as property, or livestock.

This is not how slavery was practiced historically around the world.

In fact, even ancient Greece and Rome (both of which thrived on slavery) had more legal protections and pathways to freedom. Slavery was not race based. In many cases, they had earnings and could eventually buy their freedom. In Rome they could be set free by an owner and become Roman citizens.

None of this is taught in American history likely because the ruling class worries it would hurt the average person's national pride and the ability to exert control over them. But the truth is no society can improve without honestly reflecting on their past so I hope there will be some that read this and learn.