Map of religious makeup of Africa in 1913 and 2020.
Religion as a colonial and imperial tool is interesting, it's often touted as a tool for empires to subjugate newly conquered peoples but I think in many ways Islam and Christianity limited the full potential reach of the empires and vice-versa. Under the Umayyads, the ruling Arab Muslim elite didn't make great efforts to proselytize their dhimmi subjects because it'd lessen their tax revenues as they'd be exempt from Jizya. Russian Tsars and Cossacks didn't want to convert the natives of Siberia and Alaska to their orthodox faith because then they'd be exempt from paying the yasak fur tribute. As you mentioned, both Muslims and Europeans didn't want to convert the interior African 'heathens' to their faiths in the interest of maintaining their plantation and domestic slave labour supplies. So-called Muslim and Christian empires were more about bolstering their economies than spreading their faiths.
Ethiopia Muslim-majority by 2030
Banu Umayya slowed the spread of Islam so they could continue to collect jizya. Even when non-Arabs in Central Asia converted, Banu Umayya forced them to continue to pay the jizya. Only Umar ibn Abd Al Aziz (Umar II) objected to this.
Ethnic pride, slave raiding and growing dependence on jizya by Muslim rulers slowed Islam tremendously.
Muslims ruled India and Spain for centuries. They chose to mistreat the natives and degraded them, so in retaliation the Spanish elite and the Brahmins kept alive the memory of a Christian Spain and Hindu India. This meant that Islam was unable to indigenize itself.
Muslims have been in East Africa for millennia, why are there still so many pagans near the coast by 1800? Because slave raiders discouraged the converting of Africans because that would mean that they would have to travel further and further inland to capture legal slaves.
"Muslims" have been working very hard over the past 1,400 to slow the spread of Islam. A lot of these famous "Muslim" rulers were, for all intents, kuffar standing in the way of Islam's spread.
Laagite
الداروودي
Add the Ottomans as well, they ruled over ex-Byzantine territories for over 500 years at their peak they've reached Central Europe but as you said they were mostly interested in jizya/slave trading.
