Agreed!
I think Isis would be ideal in the "mother of the Messiah" role, not least because it would allow the WOB and/or Sun-Tzu to keep her potential power over the cult they're running to a minimum. But Takiro seems to have a plot in mind where Isis has (or seizes) more power for herself directly, to which I say YES PLEASE.
Fewer hapless princesses in fiction is a good thing. So instead, I humbly offered other options, and using a Trinity is a simple and extremely popular way in history to combine all the flavors of multiple popular deities in a polytheistic religion together into a single easy-to-worship package for a unified cult. (Note: this isn't at all how or why Christianity created or uses its three-aspect Trinity -- for one, in a polytheistic religion, you can customize the specific deities/aspects as needed, even between different worshippers in the same group -- but the existence of a Trinity could still allow the Cult to try to recruit Christians as well as polytheists and syncretics)