Why would there be serfs when Africa already has a long tradition of slavery, particularly since some of these states are likely to be Muslim states and Islam condones slavery? Imperial Russia's serfdom emerged under local conditions,
As your post implies, its far easier to enslave people who are different. Conversly, its harder to enslave ones own people- or closely related people.
Imperial Russia's serf system evolved from slavery. Once Russia, Ukraine, Belarus, Balts etc, coalesced int nation states (even if imperial provinces), slavery was no longer going to socially tenable. The slaves looked and acted too much like their owners. But.... there was still a need for forced labor.
So.... wheel out the serf system. Over time, social norms developed via trial and error in the Russia Empire as to what the society would, or would not, accept regarding serfing. This body of customs and norms creates a socially tenable form of "slavery light". Needed forced labor is preserved while slavery is banned.
Going to Africa....
I like your idea of an earlier and stronger arrival of Islam leading to the rise of African nation states. As in the Russian empire, the unifying power of nation states leads to African slave owners and African slaves getting too close for comfort. Slavery is no longer tenable. Yet, there is still a need for forced labor.
No worries though, the African nation / states implement Afro serfing- a socially tenable form of slavery light. Societies can be governed by the same core concepts. African serfing norms resemble those of Imperial Russia. These norms include absolutely no serf exportation, limited transfers and no direct sales, rules regarding what serfs can be "held" by which uhmm.... "holders" etc.
When the Europeans arrive looking for slaves, they find plenty for forced labor. But..... no slave vendors and no slaves (well, no direct vendors and no
slave slaves at least).