Making a prediction of characters future stats based on how other characters are growing and how existing patterns are shaping is valid. And since statements and hype as open to interpretation what the "primary source" suggests should also be taken with a grain of salt.
That is why context is king, and there is no context that would suggests stagnation.
Kyoukai is closer to Kyou in terms of her role in the plot, but in terms of style Yotanwa seem to be better comparison due to her more agressive style. Plus we still don't know if Kyoukai is even set up to become 6GG and what her future will be while Yotanwa is already hodling the position.
I'd say she's closer to Tou in that her approach will be based on whatever the enemy and circumstances give her to work with rather than defined style or preference. At Retsubi, she surveyed for weaknesses and exploited them. At Ryouyou, she picked up the pace because of the food supply. At Hango, she hung back without overcommitting. It's a limited sample size, but I would not describe YTW's style as aggressive.
Kyou Kai seems to be Hara's favourite character, so if she states a goal, I'm of the mindset it's likely to happen. I'm not particularly invested in seeing her climb the ranks but I'm along for the ride.
Yotanwa became a leader of her own state in also what seem to be her 20's and carved a decently sized territory on the map. She couldn't become 6GG at young age, because the system didn't exist until Eisei brought it back. Like... you think it's not important that Hara gave Yotanwa the highest stats in history of 6GG? He clearly sees her as something special. Yotanwa was mostly active in a different region then central plains so her name didn't spread to the same extent, but thats not really her fault.
Revealing YTW's rank in nobility being on "on par" with a GG (I can see how but wish Hara explained it) was a nice way for Hara to signpost her level of talent, but that appointment itself was not given on the basis of aptitude. It was given in gratitude and, imo, meant to telegraph a hopeful reflection of the king Sei wishes to become.
I also think you're overstating the size of YTW's realm and what that meant prior her joining hands with Sei. I'm sure it's grown tremendously since but her terrain was considered pretty remote at least the start of the series. Even as recent as the current year in the timeline Qin had stateless, ungoverned zones with populations large enough to yield hundreds of thousands of military age, able-bodied men without tapping out the sources (meaning the actual population is even bigger).
YTW is a messianic figure to her people. I'm not doubting how special or talented she is, but I don't think her promise is hinted as being equal to Kyou, whose hype was absurd even by Kingdom standards. Never mind greater.
Same as Yotanwa. She literally defeated the strongest warrior of the mountain realm at the time (Shunmen) as a teenager or young adult and later did what was considered unimaginable before (united Xirong tribes).
An impressive feat, no doubt, but I don't put those kinds of 1v1s on the level with achievements on a real battlefield with more sophisticated strategy and tactics.
Kyou came up during the golden years of the Warring States. Not only was she claiming heads well before she met King Sho at age 16, it didn't take her long to become a general after that, or a 6GG after she hit that milestone - improving massively after each one.
Again, no one has this kind of hype.
I just don't think her talent is as good as you make it out to be. There's multiple characters in the story that are hyped as prodigies at warfare or that achieved great things at young age. For example Moubu was already hyped as having stronger offense then her in the early portions of the story. Kanki was considered the greatest man in Qin at turning the tables and the symbol of Qin before his death etc. I think there's a clear favoritism Hara is giving the current generation, since they are set up to complete the unification.
Hara hasn't paused for longer than 5 minutes to praise Ou Ki since he died so idk about clear favouritism.
Mou Bu was in his mid 30s at the beginning of the series and lacked both judgment and experience. Perhaps that would've been different in the same era, or perhaps he would've gotten himself killed. Either way, he and Tou still live in Ou Ki's shadow to an extent and I don't think either has or will surpass Ou Ki.
YTW has been sidelined for most of her tenure. Kan Ki fucked himself and died 0-1 against equivalent talent. Ou Sen is the only one I feel might be flat out better than those mfers and Hara all but outright confirmed he was on par with the original generation to begin with.
Earl Shi is the closest in terms of career trajectory. Rin Shou Jo was set up as the ultimate what if in his own way, but not so much in terms of raw talent or even intellect - I think it was his wisdom that was the real danger for his adversaries.
Ri Boku, by all indications, was a late bloomer in life and I have that mfer #1 so I don't think it really matters all that much in the grand sceheme of things, but I do think Hara was clear Kyou was different and special even by the circles she travelled in.
