This is the answer. Sugar was the magic ingredient for chattel slavery of Africans, tobacco and cotton were both profitable but the margins were always much lower than for sugar and they developed much later. The idea of extracting sugar from beet dates back to 1575 but it didn't become commercially viable until the early 19th century, if you could bring that forward by 150 years to the mid 17th century chattel slavery remains marginal.
Yes, the first we need is for early experiment in sugar production using fodder beets instead of red beet, the red coloring in red beets make the process much harder. The next thing we need are countries with a clear mercantilist focus to embrace this, I would say France, Austria, Saxony, Sweden and Denmark-Norway are good candidate for this. Especially France focusing on beet sugar would be great, as it would give it the prestige to spread across Europe.
If sugar is no longer the money maker, we will likely see the existing slave population being moved to other markets, and they rarely have the same mortality rate as sugar cane farming. The result are slave population with natural replacement rates and falling prices for slaves. This will both result in falling interest in importing more slaves, but also likely less interest in keeping close control with the slaves, it’s likely that slavery will shift to de facto serfdom and tenantship.
Of course in Africa, the falling interest in slave import will disrupt the African markets and the coastal states will be forced to shift to other goods to get European goods. So they will to go away from OTL easy access to money, which will result in stronger more developed coastal states, which is the recipe for an inland expansion. In OTL the disruption happened at the worst possible time for the African state as it happened as European improvement in medicine allowed inland expansion. Of course some of the coastal tribes may decide to become vassals of European powers, but without European population being able to survive long term in the region, local Europeanized African elites will run these states.