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

[Lauront]

I think it very compelling. Emet being willing to, and in fact planning on, sacrificing human rejoined souls says much about the lengths they were willing to go to to bring back those they lost. Likewise, you’re free to disregard it or insist that the plan was completely different in the past,

I am discounting it because it has a huge problem in it: 25% of the ancients remained at the time and the plan then was to sacrifice the new life, not that 25% or so. That 25% was sundered in the process. So even if I were to be charitable here and assume that their plan was to redirect all that soul aether to the sacrificed inside Zodiark, it'd imply a change in plan in and of itself. If not? Then we can no longer rely on that source to inform the what of what was being sacrificed. All we know is that 1) it included entities bearing souls (animals at a minimum) and 2) which Venat's faction thought may suffice to inherit the star (could range anywhere from a familiar of reasonable intelligence to ancients.)

but I think most would agree it suggest that more than small animals were going to be sacrificed in Amaurot.

Sure, but it nonetheless leaves open quite a broad spectrum of possibilities...

Yes, but Venat's words suggest that the method was also wrong. If the method was irrelevant due to the nature of the new life, then it wouldn't have to be mentioned.

Again, because she has an end-point in mind on where it would lead to. In Anamnesis Anyder in particular you can see that she grants that the Convocation means well and aims for the same as does her group, which is a prosperous future for their star. Of course in EW she's aware of what Meteion's report contained, so that's why she's stressing the value of suffering and avoiding sacrifice to reclaim life as it was.

In that case, they wouldn't need a living sacrifice, merely a sufficient concentration of aether.

At the end of the day, he's a primal. They function on aether. Souls are made up of aether - possibly a very big hit of it. Given how vague it all is we can but speculate... and again, let's say it specifically required souls, again the possibility here is that a high number of weaker ones could logically suffice to substitute out a smaller number of more potent ones. Part of why they even had to sacrifice their own, in the first place, is that the Final Days had starved them of all other possible sources of aether due to its rapid escalation.

Venat was an Ancient like the rest of them. When you have power like that, every decision affects the fate of others. It took only one depressed Ancient to almost destroy the entire universe. If a bunch of Ancients had the right to sacrifice a bunch of unborn, unwilling lives to Zodiark, Venat also had the right to do what she did.

Hmmm how does that follow? You've no real awareness of what these lives were. As I said, it's a broad spectrum of possibilities.

They were planning to sacrifice more to zodiark before Venat's faction decided to create hydaelyn to counter and stop zodiark and his followers. Emet even said that the sacrifices would not be in vain because they are hoping Zodiark can bring back all the ascians that were sacrificed to it. That was why they wanted to create the rejoining, so that they can continue where they left off and power up zodiark to defeat the whatever the finals days was and bring back their people to life.

Edit: They actually showed the scene of Ascians, in their despair, sacrificing themselves to zodiark in the hope of averting the final days which finally made Venat sunder the world. I think they create that scene specifically to show one part of Venat's reason of sundering the world.
https://www.youtube.com/watch?v=qlA0lGrSRqM

This is why Emet said to our character at the end that the "past they were looking for is not the future we are building towards to" Emet understood already that their way was not the best way to defeat the final days and for the future. But he is duty bound to do what he planned to do before as the holder of the title Emet Sech. But deep inside he knew we were the best chance against the final days hence even back in shadowbringers he helped us against Elidibus to "put his friend to rest" and put his trust on us.

Yes, that's why that poster is asking about more than the 3 - the third was the planned but never materialised stage. Also that scene isn't great, because it's stylised in nature. If you want to refer to the sources on this detailing the sum total of sacrifices and what each was for, see here.

What Venat did was just as terrible as Hermes, we just need one line of her stating it and we are all good. Just one line. No need to dwell on anything like "necessary evils" or anything to keep with the theme no , just skip it. Give us more stupid rabbits.

She states it at least, by stating what she did was neither just nor kind, but what annoyed me more is the lack of any ability to rebuke her for it or at least give her the cold shoulder over it.