I mean at the end of the day the Straw Hats only will be as strong as Oda needs them to be to defeat the enemies he plans for them. So Sanji will be as strong as he needs to be to defeat YC2's and (most likely) whoever is the 3rd-strongest admiral. I don't really see any YC2's having CoC and I highly doubt that every single admiral has it. But if Oda thinks he will need it to defeat the enemies Oda has in store for him then he will get it. Do I think it's likely? Not really, but that's just how it works.
It's the same thing for Luffy and Zoro. Luffy will be as strong as Oda thinks he needs to be to defeat Yonkous and probably Akainu and possibly someone unknown like Imu. Zoro will be as strong as he needs to be to defeat YC1's, probably whoever the 2nd-strongest admiral is, and Mihawk on top of that. So them having access to ACoC makes sense if they are having to fight those kinds of enemies, especially since we've already seen 1 YC1 who has CoC in Katakuri.
I think a lot of people seem to operate under the assumption that Oda always has the M3 gaps in mind when writing in fights and power-ups. However, I don't think that's the case. Oda likely writes with the whole series in mind, and probably doesn't care if the gaps look consistent. That's why sometimes it looks like Sanji is falling way behind Luffy and Zoro, sometimes it looks like Luffy is light years ahead of Zoro and Sanji, sometimes all three of them seem pretty close, etc.