A lot of stuff to unpack there. Firstly, as I've explained countless times, Katakuri barely lifted a finger vs. Ichiji, because Ichiji tried to blow himself up to take him out for protecting Capone/Caesar. The SFX in the manga show this, and so does the anime. All Katakuri did was avoid his explosion. In essence, Ichiji got desperate to stop Katakuri, and pulled a Pedro vs. Perospero. However, Reiju/Ichiji are more than capable of fighting a Smoothie level fighter as shown by manga/anime. They are clearly YC3-4. And again, Katakuri vs. King, is like Oden vs. Marco. Katakuri was made the anti-Luffy, not the anti-Marco. King is not on his level, who is most like the anti-Zoro.
In reality base Sanji vs. King should be like Marco vs. land-Jinbe/pre-Yami BB. So a classic "YC1" vs. a "YC3.5-4". Anywhere from a mid to high diff = More than capable of Sanji holding him back. With a RS, it should be a coin toss though.