Personally that's exactly the reason why I find the strength progression route Oda's going with Sanji so intriguing.
You know he's defo going to hit certain heights (i.e. will always be relative to Luffy and Zoro) but unlike those two you're not exactly sure what route Oda's going to take to make him get there. Even just from the perspective of being able to hurt Imu's Zombies, you kinda already know how Luffy and Zoro are going to counter regen (i.e. better mastery of CoC) but how Sanji will be able to do it is still a mystery. What is certain though is that like those two he will also be able to do it, be it via CoC or something else entirely.
What I'd say though is that in the latter case, if better mastery of the CoC, the ability of the so called "special ones" is the only ability that is able to hurt immortals and Sanji is able to do it in a different way completely unique to him then what does that make Sanji then

. Moreover, we gotta start having the conversation about how said ability compares to the ability of the so called "strongest" according to the CoC merchants on here as it'd effectively able to replicate the same result as it.