You speak as if izanami needs to be used right after its been prepared. It doesn't.
I do. That's how Jutsu works. Molding Chakra according to the technique that wants to be used, then it activates. Exceptions are time delayed Jutsu like Zetsu's spore technique. Other than that, I don't see any requirements. What should he have prepared beforehand? It's just like Izanagi, Koto Amatsukami or Tsukuyomi - they don't need any preparation aside from the conditions they are used under, for example eye contact.
Whatever Itachi spoke of when he said 'It's already prepared', it was related to Kabuto - nothing he could've done before. May it be making contact with him, eye contact or merely looking at him from close range.