Ahhh, where is it stated Oshtur transcends godhood itself?
I would argue the Vishanti are more powerful than your single Elder God but Oshtur being more powerful than her other siblings is never stated.
Oshtur can certainly be grey, after all she is also not allowed to directly operate on Earth just like Chthon and Set. There is also the implication from Marvel Tarot that Hoggoth is an Old One, a kin of the likes of Shuma-Gorath. Yet Oshtur has him as company and part of the trinity.
Also Agatha doesn't have a significant patron god I can recall, she probably worship your average pagan gods but no one major.