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

[Kaurhz]

It has pacing issues, sure.

But to address some of the points here to the best of my understanding.

Zodiark was still sundered, so he wasn't as powerful as he was on his original summoning prior to his sundering. The only thing that happened was the seal on him was broken. This shattering only broke his imprisonment. He was powerful, but not nearly as powerful as how the unsundered know him to be. This point is also a little bit weird, because the argument could just be made of "why didn't they just do this then to break his seal, instead of the rejoining escapades" - and I think the key difference here is that Fandaniel wasn't concerned with the methods, just the result, to which his methods could have potentially prevented bringing back / restoring the mankind that they knew.

I'd have liked it if there was more on the causality of the final days as opposed to it being a projection of emotions on the celestial currents (as I understand it).

We still don't know the length of time between Meteion leaving the planet and when the final days started to hit the planet(?), not only that Emet-Selch had openly admitted that their methods would not have carried mankind so far. The ancients weren't concerned with solving the fundamental cause of it as they were with just delaying the inevitable and feeding Zodiark to resurrect the sacrificed. Plus, I am pretty sure that the ancients didn't have the ability to manipulate/manifest dynamis given how dense their own aether is, and to my knowledge it is the abundance of aether which drowns out dynamis. The ancients were so crippled by their own despair that even trying to fight against dynamis or manifest it would have been nigh on futile.

At least this is the perspective I have on the story, and I think with her actions, and the 'reasoning' for them is precisely why she's ultimately a well developed character.