What's the point with that quite lackluster story in Endwalker?

[Kozh]

So that means even herself could have somewhat helped if she had really intended to instead of sitting on her high horse, instead of becoming a 2nd judge in the "judgment of humanity".

It feels like pre-Kairos Venat and post-Kairi Venat are written for different story direction. During the Hermes boss fight in Ktisis, she said "this world (the Ancients) is not perfect, but I'm going to save it, warts and all". Not too mention the "there's nothing impossible". Meanwhile after kairos, Venat basically went "you know what, nevermind that lol."

Did he one day wake up and came up with all of that on his own?

That's the power of being the writer's favorite character.