On a side-note, slavery has meant wildly different things at different times in history. For a very long time, the world had no real merchant class. The world was divided purely into the aristocracy and everyone else. When the world worked that way, slavery was an iffy proposition that faded into and out of existence. The advantages of owning slaves versus simply oppressing free peasants was dubious and situational. You have to house and guard slaves, after all, which can cost just as much as paying peasants shit wages.
There was also very little racism in the time when the world was just aristocrats and peasants. Egypt, Persia, and Rome were all multi-ethnic. That's not to say that peasants weren't racist, but they had no power and no influence. Aristocrats eschewed racism because it got in the way of their empire building. If a white aristocrat from the northern part of the Roman empire saw an advantage in marrying an aristocrat from the black African part of Rome, then they married and fuck what the peasants think of such things, the combined family now had holdings in both the northern and southern reaches of the Roman empire.
Because racism had almost no part in aristocrat-era slavery, slavery itself often wasn't very oppressive by the standards of the time. Slaves occasionally even had more rights than free peasants, because their education and training made them an investment that you didn't want running away.
Two things changed that made the African slave-trade so much more egregious than previous forms of slavery. One was the rise of the merchant class. You now had peasants with real money. Real money but not enough land to make playing the let's-marry-to-fuse-our-holdings game a big factor in their lives. That meant that the bigotry and provincialism that had been irrelevant in the aristocrat-era now had real power, and there was no financial motive to put it aside. It ran rampant, and became a reason for peasants to think themselves superior to at least someone somewhere. The aristocrats had never needed that psychological boost, it was enough that aristocrats were superior to peasants. So what if one aristocrat was white and the other black? You were both aristocrats first and foremost.
The other factor was improvements in sailing. It became practical to relatively quickly travel and ship things long distances via water. That meant that these bigoted, provincial peasants with money could now easily travel to places with people who looked and sounded nothing like themselves.
The combination proved incredibly ugly. Bigoted, provincial peasants trying to compensate for the inferiority complex they'd lived with their entire lives now had someone they could dehumanize utterly for profit, and the means to ship people on an industrial scale.
Wakanda seems unlikely to have experienced that particular combination. While Wakanda almost certainly experienced the rise of rich peasant merchants at some point, their isolationism probably meant that those merchants didn't have easy access to large numbers of people who didn't look or sound like them. The result would have been closer to China in the same period than it was to what you saw in Europe.