I don't think they are trying to expand the theater of war.
I think they are doing what they have been doing all along, which is to act as the muscle for Russian resource extraction.
This usually happens at the invitation of a President (either openly, like Touadera in CAR, Goita in Mali, or covertly as we see with Hemedti in Sudan).
The fact that within a day of the "Wagner rebellion attempt" in Russia, Lavrov was already reassuring African nations that nothing was changing with the Wagner stuff, is pretty telling.
The long term goal is possibly state capture - again probably to permanently secure minerals - but in the meantime, Niger has plenty of uranium for now.
The mainstream news media has started giving Wagner's African presence a much higher profile since Ukraine.