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

[Theodric]

Her dialogue in Elpis expressed that she would avoid bringing the subject with Convocation members so it felt like she avoided them at first and did things on her own. At a later point, around the time of Zodiark's summoning she made contact with the Convocation to stop them from sacrificing half of the replenished life to bring back the first batch of sacrificed.
Also, the ancients were masters of creation magic, pretty much anything could be created from it so it's not so unreasonable especially if someone such as the revered Venat was to say it.

Have to disagree with this part. No reason why she couldn't have used one of the many methods the Ancients have shown for reviewing memories. Very likely that they would not have believed her if she just told them without providing evidence but there were ways of getting said evidence.

To be clear, by 'their own accord' I'm referring to them coming to the conclusion of Meteion's existence on their own. Venat should have told them what was coming to forewarn them, though she did not do that and instead left them scrambling about in a desperate effort to try and figure out why the Final Days were happening.

Then when they found a solution to the problems that the Final Days brought about, she decided to blame them for not finding the 'right one' and resorted to the Sundering. All without giving them any explanation.

In the end, it seems that the writers decided that the Sundering needed to happen no matter what, though they were probably hoping more people would just go along with all the cool, flashy cutscenes and not think on the implications of what the context and consequences actually entailed.