Question: is isshiki stronger than kaguya?
Answer: I think Kaguya was originally weaker than Isshiki, but became stronger due to the chakra fruit. At this point, I'm feeling inclined to say that it was Isshiki that Kaguya really feared, not Momoshiki. So, yes, at full power, Isshiki can be stronger than Kaguya and Momoshiki.