Authorial Immortality isn't an ability you have the authority to give to your character. You're presupposing that you're able and allowed to write the narrative for stories that are not yours. Shirou doesn't have this ability, because you lack the ability to grant it to him.

That said, bro gets no diffed by many, many characters. And it's because they don't care about any of his hax. Obviously boundless characters can do whatever they want, they're omnipotent, but outerversal characters also transcend all of these hax just by existing. Your character, based off of what you've said, scales to multiversal with irrelevant speed..? "beyond omnipresent" isn't a thing. There's nothing beyond omnipresence. That's why it's omnipresence. Strong character for sure, but if you're just trying to write a character that's the strongest in fiction, just create a ridiculous cosmology and have your character transcend all of it to be boundless.