Fs will play as much a part tho, because it will allow him to dodge kaidos attacks. Because if they connect then its goodnight sweet prince. The same reason law is there btw, it’s to top people in and out of the fight with kaido. Actually he wasn’t able to do it well after the big mom thing and he had to train for it but you know luffy, once he does something and gets the feel for it he will learn it in due time.
Against kaido luffys didn’t use fs because you have to be calm to use it remember, and he was the farthest thing from calm back then.