Luffy is fully fine tuned. He has fought speedsters, FS users, and the WSC, he should be ready for Kizaru. He's had a pitiful display thus far not a single scratch on Kizaru yet, idk maybe Kizaru is just broken afterall, where only G5 can do something.
I will tell you know don't bring up FS we not talking about shanks we talking about luffy.
Where first thing that happen in Kaido fight was he could not dodge TB fully even with FS and had to take hits the whole fight .
Shit kaido copy his FS and dodge all his snakeman attacks in a fucking miles long dragon form .
FS is trash unless we talking shanks , luffy can't find tune shit if keep get his ass kick out of it .
Also it was expected lufffy would need G5 luffy vs Kizaru , he only uses other forms vs Kaido only cause he did not have G5 yet .
Most of that fight with Kaido fight was getting luffy able to match in him after all those KO and power ups.