I like the idea here, though like you said 5 new monster girl races could be tricky. I don't know how much is out there to pull from, but my first thought we be something "Fallout"-ish, like Mutants. You could also maybe do some sort of "stranded alien" special matriarch that could fit a wasteland type theme, which means you could pull from all sorts of sources like Mass Effect or Star Wars/Star Trek. Another monster girl idea could be feral humans that had regressed somewhat genetically to make them almost Planet of the Apes-like.
From a lore standpoint you could almost do a blend of different post-apocalyptic ideas. While the desert is already supposed to represent a "lush area turned wasteland by the fierceness of the fighting", you could build the Wastes as an area that was devastated by the use of a truly horrific weapon, one used in desperation to avoid utter defeat and instead twisted that land and its residents to a mockery of its former self. Sort of a "Pre-Curse" weapon that everybody afterward agreed was too terrible to ever use again. Think of it like the use of an Atom Bomb, while the curse is more related to a terrorist-released bio-weapon haha. This also meant that it was avoided by most life until some of your released daughters decided to try to tame it.
So there's 2 ideas for regular monster girls and 1 for a special matriarchal one, as well as a lore possibility. Hope they help.
P.S.- Personally I'd use Anna Bell Peaks for the human matriarch, but that's just personal preference
