I like it, ty. Will start working on this idea now. However, what if Yuan-Ti are the Rebel Folk (rebelling against snake brainwashing, opposite of humans) and Shadar-kai are the sixth folk (wise, from another world, like Ents or Eagles in LotR).
Yeah, totally can work.
If the Yuan-ti are the Rebel Folk and the opposite of the Human Jock Folk, this says something about the Jock Folk who personify the quest of the story.
The Yuan-ti are Fiend (and Fiend-aspiring) with powers to charm and cause fear. That and their snake traits, tap into a satanic dragon vibe. Both instinctive and spiritual terror, as well as seductive and corrupting. As Rebel, the Jock is confronting and utilizing this darkside.
This would make the contrasted Jock Human more Celestial-aspiring, with resoluteness of mind, honesty, wholesomeness, and realism. Not just surviving the apocalyptic fallout, but striving to build a utopia. Then the Celestial Aasimar as the Heart Folk, would tap into religious inspiration, maybe an angelic prophet figure, a vision of a hopeful future.
If the Shadar-kai are the Sixth Folk, then their Shadowfell deathliness means death itself is the villain. Perhaps, the apocalypse resulted from a kind of nihilism. The villains might have a movie Pitch Black necromonger vibe, that preys on the souls of Humanoids as part of empire building.
Note that the setting invites a fundamentally Psionic vibe. The Thri-kreen, and Aasimar as old-school Angel, and the Yuan-ti are all innately Psionic and telepathic. The Human includes psi, but not all would need to be. Even the Warforged might be Psiforged and innately Psionic. A crystal-wielding Psiforged would give the Artificer a Psionic flavor. Maybe check out the UA Artificer with the Archivist as a Psionic subclass, with a psicrystal as the "iphone".
In this context, the villainous Shadar-kai might be normally non-psionic, and more Undead oriented. Albeit ghosts too as a disembodied mind or an echo of a deceased mind might be a Psionic phenomenon.