Your map also woefully simplifies the African religious context. I'm pretty sure that Zambia and Zimbabwe are at least 70% Christian.
Also, syncretism =/= indigenous belief =/= syncretic religion. Madagascar's religion, for instance, is a type of veneration of ancestors which is also practiced by the local Christians. It doesn't necessarily means that they have an organized syncretic religion per se, like Santería. It's pretty hard to truly define the strength of each practice in this type of society, however, for sure, Chrstianity is the "religion of prestige". It's pretty much the same reality that we find in Shinto/Buddhist Japan or in the Americas with Christians who practice Santería, Voodoo, Candomblé, etc.
I'd say that it's only around in the gulf area we can still find "true" native religions, but it's just a wild guess. Most of the African countries don't have reliable demographic sources anyway...
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As for the topic, it can be easily achieved in the 19th century: Earlier end of the slave trade and more countries competing for Africa generates a colonial rush (IOTL throughout most of the 19th century the UK and France were the only real players in Africa). Remember that France only invaded Mali in 1883 and Sokoto was only conquered by the British in the beginning of the 20th century. If all West Africa was in European hands by the early 1800s Christians would be an important minority (or even a majority) in the Muslim Sahel by now.