The costs to him wouldn't just be metaphysical strain, it would permanently damage all that He is to those He inspired - if his students knew that, if faced with any really important problem, like the destruction of the multiverse, Irori would be standing above them like a protective parent, ready to take up their task if they failed, even after they told Him to stay out of their way. Irori certainly wouldn't hide the fact, if it became a fact that was true.
To take Carissa Sevar's task away from her, or even prepare to stand ready behind her, would deal an unhealable wound to that-which-is-Irori-Himself.
And so what? Protecting all of Creation is a good reason to hurt Himself.
But Carissa's thoughts have touched on tropes, if not centrally, and Irori has bought enough information from Abadar about Osirian prediction markets on a very confused topic, seen enough Himself about how probability twists around Carissa Sevar and godly actions don't have their god-expected effects, to fear/hope/want-to-accept-in-accordance-with-His-nature that to save the world from Keltham is Carissa's task and not His. That forces above even Pharasma dictate that success or failure is down to Carissa alone.
She believes and feels that way, and it blazes out to Irori in a clear light bright enough to shine across a thousand years of distance.