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

[Lauront]

Interesting. Though even from the text it appears more of a promise that through sacrifice Zodiark would be able to renew the star through the same magics that are seen in Elpis. I wonder what the guarantee was of this plan being successful, and how did they plan on dealing with a Primal on the scale of Zodiark. Plus, would the potential tempering that came with a stronger and stronger Zodiark ruin their plans either way? Not to mention the threat of Meiton was still out there, so even if life was created anew, what would be different given The Song of Oblivion's presence. Mind you this is all moot due to the fact that the world HAD to be sundered for us to exist. I assume the thought process was that the ancients who presumably had not suffered and hoped on a constant basis that their sundered counterparts did would have not been able to stand against Metion, thus falling on us(and Zenos lol, though I actually liked that part).

On tempering I'd recommend you give this a read. It's ultimately a red herring, AFAIC, and mostly inserted to explain the Ascians 12k years later, because they still need to explain why e.g. Lahabrea and Nabriales acted the way they did, back before they had the SHB plot in mind. But with the primal 1) not summoned via the flawed rites the Ascians taught the beast tribes and 2) lacking a will outside of its controller, beyond perhaps self-defence instincts, it's hard to see what role it'd play, and it's given zero story significance at the time. The conflict divides the Convocation as well (touched upon here if you go to the French localisation of the SoS scene), which pushes against the hivemind interpretation of the issue many have, so provided that they acted as Emet did, and preserved their memories and identities, it may well have never become an issue for them.

Regarding whether Zodiark would succeed at restoring the star, he did succeed at that, as the second stage was a fait accompli, and succeeded in renewing the cycle of life. It's the third one which was not completed, but in all honesty, I see no reason why it would not, as the souls were preserved inside him.

The issue regarding the sundering is she 1) believed they would eventually go on a path similar to the Plenty and 2) were inherently unable to easily manipulate dynamis. 2) is debatable, because the story itself introduces a number of possible workarounds, including no less sundering - just not the entire star or race of beings, but on a more selective basis. There are other possibilities, like deploying familiars or other arcane creations without the design flaws of Meteion, exploring how the suppression field in Ktisis Hyperboreia could enable them to hone dynamis manipulation, etc. Zodiark's shield persevered for a minimum of 12k years, so it would've bought them plenty of time to experiment. 1) is also debatable, because they were not presented with concrete evidence of what was animating her concerns - she constrained what she revealed. I agree with this poster's interpretation of why that belief might've come somewhat a little too easily to her.

Emet described what the sundering did in the demonstration with Ryne: "Two individuals, identical in appearance, yet reduced in all respects. Strength, intelligence, the soul itself - all is halved." Externally they may have looked the same, but they were 1/14 the person they used to be and devoid of their memories.

Also, the sacrifices were never the real issue. Based on the LL, Venat always intended to sunder the Ancients because she believed (her personal opinion, not objective truth) that they were incapable of change or of defeating Meteion. She needed both Zodiark and for the star to be restored, which is why she didn't intervene in those sacrifices. The third one didn't matter in regards to her plan. I don't believe it's explicitly stated, but given that Elidibus had exited Zodiark in an attempt to mediate and Hydaelyn chose to sunder Zodiark at that time it may have been a calculated move on her part. I certainly wouldn't put it past her to have orchestrated the divide in society to allow for such an opening, especially given that her followers in Anyder seem oblivious to her true intent and the subsequent consequences of it.

Agreed - and I believe they may have re-considered that stage until later if presented with good reason to do so - i.e. the evidence about the Plenty and Meteion.