I am suspecting that there isn't a cherub species, just the polite fiction of one, and our dear Calliborn/Calliope are a creation of paradox space in entirety.
Likewise, I suspect that a "proper" Sburb session is a fictional ideal that does not actually exist. The Beforus session failed, though it was closest to normal. The Alternia session was technically successful but then Noir snuck in and wrecked the place before they could claim their reward. The Earth session was null, and thus unwinnable, and the Earth-2 session is void, and therefore even more unwinnable. Then you got Calli's dead session which is so unwinnable it's actually a secret hardcore nightmare mode.
But, that's it. I am making the prediction that there are no other Sburb sessions. Paradox space is full of these sessions and their various timeline offshoots and nothing else. Sburb will be the ultimate ontological paradox.