Russia is in it for reasons that go beyond Wagner paying themselves with local ressources. The migrant crisis of 2015 deeply divided the EU and the members have since shown a tougher stance on migration, Erdogan's Turkey playing a game of blackmailing the EU about potentially opening migration routes to the EU is something which showed Russia that controlling migration routes can be some sort of leverage to be used against the EU.

So Russia is trying to gain presence in Africa for 2 reasons linked to that: 1. With influence in countries migrants leave from/go through with hopes of reaching the EU, they'll have a card to play to get something from the EU in exchange for their cooperation with whatever migration policy the EU will want to put in place, or straight up go against that policy to try to destabilize the EU. And 2. If these countries stay unstable enough, this will keep a steady stream of potential migrants to the EU open.

TLDR: Russia's Africa policy isn't really about Africa, it's about the EU's migration policy.