If by the others you mean Suzu and Ryutarou, they don't have Afterstories keys, though we could feasibly give them that, idk if its worth it though.
I mean obviously in a fight Shizuku would beat Endou, he only becomes a issue for the wives where they are constrained in a situation where they can't just wipe out all the clones at once, but that's a little different from saying she is more skilled than him.
As for Shizuku's zenith, she cheated using soul magic, while Kouki did it with pure will, Shizuku is definitely skilled, but at this point, we haven't seen enough to know how much more skilled she is in the Afterstories, by feats i think Kouki edges her out right now.
I want a Shizuku arc too.
I was talking more about Kouki and Endou since you could probably make the resistance part look better, and the keys to Suzu and Ryutarou probably are a good idea, they don't really have many feats in the after stories but just the new resistances with should make it worth, the AP would basically be 'stronger than before'.
I mean, Endou have showed that he can hold his ground against Hajime and characters of that level. Endou is really skilled, yes, but what make him really dangerous is the combination of his presence, numbers, absurdity and tenacity, so in pure skill Shizuku is definitely above him.
She cheated yes, but the fact that even characters like Kaori or Shia are unable to cut curses or punch them despite having the same level of soul magic (in the case of Kaori she is far above) show that indeed is a skill feat. But yeah, by pure feats Kouki edges her out right now, but with statements and this little showing they should at least be equal.
Though, regardless of who is more skilled between the two the fact still remain that the skill level in Arifureta is crazy, the fact that there normal humans like Allen, Fukube and even Sabastian (the butler of the princess in the Sky Dragon World) show that (heck even Vanessa, who I hate btw, have a really high skill), if you think about it basically all the current profiles scale above that level of absurdity, like Gahard and Meld scale above them since they had the titles of the strongest and even after meet Allen, Fukube and Sabastian the group of Hajime still respect the skill of both of them.
Then you also have the Haulia group who could win against the empire group and even when facing Gahard, the Haulia at the same time are equal with the Yaegashi group who at the same time defeated the elite forces of other countries when they were brainwashed by China group, which mean that they probably defeated elites at the level of Allen and Fukube since the contries literally wanted the returnees at all costs.
And then main cast scale above both groups, the only exception would probably be Ryutarou since he don't really have feats in the after stories.