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It's important strategically for Russia and they can sell arms like you say. 
But also unlike the west, Russia does not spend billion sending huge amounts of high tech material to intervene in a region. They send a few guys in a hilux and maybe they send a BTR or a helicopter(though I think even then they might be using the equipment of the host nation and not providing it themselves). On top of that they then get gold mining concessions and fund themselves and even make a profit anyway.
So it doesn't cost Russia much if anything to do what it does in Africa.
We also don't know how much involvement China has in this behind the scenes, because China stands to benefit from Africa hugely. They could even be funding Wagner as well.

Russian influence over the region is not good for the US as Russia can use the Sahel to project power in other directions, like southwards into centra Africa, east into the Gulf of Aden, North into the Med and even west into South America as the distance is not that far.
Plus the benefit to China outweighs any benefit annoying France and the EU could have.

Russia could probably bring more stability to the region and it could easily bring enough to satisfy China, they don't care about human rights or democracy or anything.