It was pretty much a given that Sanji will not be revealed as a CoC user before Zoro, as much as Zoro was not revealed as one before Luffy.
I don't think anyone would question that as far as overall Haki/Ambition goes, just as is the case with overall strength, Zoro would stand above Sanji, and thus Oda would showcase his awakening first.
There's also the fact that Zoro is almost 4 months older than Sanji, so all things equal it would still make sense for his achievements to take precedence ? Though this is less of an argument.
I think the SH's who will get CoC are the one's who walked into Wano with their Haki already awakened, so that's Luffy, Zoro, Sanji and Usopp. I also expect the order in which they get CoC to account for how early they awakened their Haki. So you have :
Luffy with CoC, CoA, CoO already confirmed as soon as the time skip started.
Zoro and Sanji with CoA and CoO when the time skip started, and Zoro just getting his CoC confirmed in Wano.
Usopp as the only other SH to awaken his Haki during the time skip up to this point.
Given this pattern i expect Sanji to be next in line after Zoro to get his CoC confirmed. As for when, it could still be in Wano to go hand in hand with the above pattern where in Sabaody Zoro and Sanji were confirmed with both CoA and CoO. It could be then that in the same arc they also get confirmed with CoC.
CoC clearly plays a significant role this arc, being revealed as the required tool to defeat Kaido, as well as something Kaido talked about on more than one occasion, where he said that there should not be too many Conquerors, with this mentality likely sterning from his past days as a member of the Rocks Pirates.
It's interesting that Kaido's way of handling this issue is to crush the will of the Conquerors he meets, and attempt to force them to join his crew. We've seen him trying and failing to do that with Kidd and Luffy, but i'd not be surprised if we end up seeing examples of Conquerors who's will he managed to crush in the past and force into submission.
Here King and Queen could come into the picture. I'd not be surprise if their Royal monickers also serve as a clue to indicate that they are CoC users, the one's that Kaido managed to force submit to him.
We've seen Queen's reaction to CoC and how he laughed it off when Luffy used it as if it's no big deal, not even giving his now trademark shocked Pikachu face.
We've also seen a big distinction between the way King and Queen act towards each other, and the way they act towards Jack and Jack acts towards them.
And this could serve as a clue as well because of how they stand up to eachother as fellow CoC users, while they dominate Jack who lacks CoC.
Oda could then use this as a paralel for the different means Luffy and Kaido managed to get such individuals to follow them and solved the Rocks dilema where they were fighting eachother all the time, and the purpose of this paralel could be to show why Luffy's way is the right way, and what benefits there are with Luffy supporting the ambition of the CoC users that chose to follow him.
If this is the case, Sanji's CoC could be revealed in his fight with Queen, the moment the two get to the point where they have a CoC clash as we've seen with Luffy and Doflamingo for example back in Dressrosa
If not in Wano, then my next best guess would maybe be the Vegapunk arc. Then the experiments Judge did on his kids could be brought back to discussion, and the narrative could be that Sanji being a failed experiment is what saved the lineage of potential Conquerors that has it's roots in their ancestors who at one point Conquered the whole North Blue. This could also give even more weight to Sora's actions as the one who made this possible.
Sanji could be framed as the true inheritor of their will, and the one who pushes Germa back to the right path that Judge had strayed from, which caused him to miss the chance of awakening CoC. Something about believing in yourself vs thinking you need to change yourself and make experiments to create better versions of yourself to achieve your goals would probably be what Oda goes with.