I think some people fundamentally misunderstand what CoC is and does. Here are some in-canon explanations
(1) You cannot "train" your CoC to be stronger. You can train to manipulate it. That's it. CoC is a reflection of your base strength. So this idea that Shanks has stronger CoC but he is weaker than other characters like Kaido or Mihawk, is nonsense. Shanks can learn to manipulate it better, but the strength of your CoC is fixed.
(2) ACoC is a not magic pill that automatically makes you Admiral/Yonko level. You can still be close to High Tier with ACoC (Zoro/Yamato).
This comes from VIZ exaggerating their translations. (They did the same for Future Sight)
The actual Japanese does not use the superlative "strongest". It simply says "some strong people" can use. Rayleigh said the same about Future Sight, and VIZ again translated it as "strongest".
The reason Luffy became so powerful with ACoC is because Luffy's base was already Top Tier. Just because awakening made Luffy Kaido's equal, it doesn't mean awakening will make Kaku Kaido's equal. It's all about your base.
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My main point here is, CoC or ACoC is not some magic bullet in and of itself. It's a reflection of your base strength. Having CoC or ACoC in and of itself is not going to make you Admiral or Yonko level.
(1) You cannot "train" your CoC to be stronger. You can train to manipulate it. That's it. CoC is a reflection of your base strength. So this idea that Shanks has stronger CoC but he is weaker than other characters like Kaido or Mihawk, is nonsense. Shanks can learn to manipulate it better, but the strength of your CoC is fixed.

(2) ACoC is a not magic pill that automatically makes you Admiral/Yonko level. You can still be close to High Tier with ACoC (Zoro/Yamato).
This comes from VIZ exaggerating their translations. (They did the same for Future Sight)

The actual Japanese does not use the superlative "strongest". It simply says "some strong people" can use. Rayleigh said the same about Future Sight, and VIZ again translated it as "strongest".
The reason Luffy became so powerful with ACoC is because Luffy's base was already Top Tier. Just because awakening made Luffy Kaido's equal, it doesn't mean awakening will make Kaku Kaido's equal. It's all about your base.
------
My main point here is, CoC or ACoC is not some magic bullet in and of itself. It's a reflection of your base strength. Having CoC or ACoC in and of itself is not going to make you Admiral or Yonko level.