I have doubts Big Mom will be at Elbaf. So, let's say she gets out of the volcano the exact minute Luffy lands there, first she has to get off Wano, which may be even more difficult as Hera and Prometheus seem to have vanished, so if she can't find them, then she has to hijack a ship and sail back to Whole Cake. Then, there's the whole issue there with Blackbeard's raid. And then she has to sail out there to Elbaf, as well.
So, unless Elbaf lasts, in-story, a week or two like Wano did, that's not a whole lot of time for her.
