I don’t think there was any way for Oda to logically write Sanji defeating King. He’d have to cross so many lines….. Sanji would need Luffy Treatment to beat King.
Current Sanji high to extreme diffs King.
He has the speed, AP, and reflexes to outspeed counter and defeat him if King ever uses his speed mode.
He has the intelligence to understand kings abilities quickly before they become an issue
King doesn’t have Sanjis speed or his strength or AP.