For example, I’ve heard of people who tried to argue that the history of racism in the US isn’t a big deal because Epictetus was a slave, but he did pretty well in life.

This is a pretty good example of an etymological fallacy, similar to our modern usage of the word stoic differing, as you point out, from the original use of the word.

Slavery, at the time of Epictetus, wasn’t based upon notions or perceptions of race. In fact, it would not be so based until the advent of the British Empire. The term ‘white slavery’ is a brutally stupid misreading of the history of slavery. Anybody could have been a slave in ancient Greece. Anybody could have been a slave in ancient Rome, including Roman citizens had they been conquered by Carthage…

Epictetus was born in Phrygia — what is modern day Turkey — and was likely enslaved as part of the Greek conquest of Phrygia: put another way, he (likely) wasn’t enslaved because he was other (racism) but because he was there when they pillaged it.

The moral corruption of America lies not only in the market of buying and selling human beings but stacking atop that notion the noxious idea that a specific type of human is deserving and worthy only of slavery.