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

[tokinokanatae]

So opinions does anyone feel that this suitably answers why she made her decision?

To me--and I'm sure others feel differently--there's a huge disconnect in what you outlined. I'll try to explain as best I can.

Meteion is the threat to Etheirys and the Final Days is what she's going to bring to Etheirys. You would think, then, that the thing that should be on the forefront of Venat's mind, what she is working towards, is stopping Meteion before the Final Days occurs. If the Final Days doesn't happen, then Zodiark doesn't exist.

Now, it's possible that she attempted to do this by herself, failed, and had to allow Zodiark as plan B--but there's no evidence of that in the game. What appears to happen is Venat told no one, gathered followers to oppose Zodiark (not Meteion), allowed him to be built with the lives of the people around her, and then (fundamentally) stole him for her own purposes.

Then in the 12,000 years that followed, she fanned the flames of discord between her blessed and the Ancient survivors. Surely, by the time the world was Sundered, at the beginning she could have told Emet-Selch, or Elidibus, or heck, depending on what he was like before he went nuts, Lahabrea what her plans were. Hermes was no longer there as a threat, having fulfilled his purpose to assist with the building of Zodiark as a dynamis shield. If they were still too infuriated by what they saw as betrayal to ever work with her, at least they would have made that decision while knowing there was a larger threat growing stronger and stronger as the centuries passed.