Katakuri's FS is countered by Sanji's speed just like Snakeman worked around it. Yes his defence is tough because of shape-shifting but i believe Sanji has the speed to hit him. He can arguably burn down the Mochi with which Kat would try to pin him down as well.
Um, no? Katakuri immediately dodged the first thrown punch but he was hit because Snake Man is able to change the trajectory mid-air whilst accelerating up. Sanji has speed feats of blitzing Queen. Snake Man's speed feat is against a FS user and Kaido who even had to use FS to counter Snake Man's speed. Snake Man is superior.
Also, Katakuri didn't need long to adapt to Snake Man's unpredictable fighting style and speed and at the end, he started dancing around. Bro, Sanji will still hardly hit Katakuri.
Katakuri has own fire based mochi attack. I don't think it's efficient to burn a Power Mochi when Katakuri just gets more firepower from his punches.