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

[Lauront]

That particular part of the plan comes after Emet says the Rejoining has been completed. He’s not talking about sacrificing sundered, but rejoined souls. So either he, in his own moral system, is ok with sacrificing beings worth of moral consideration because he’s bitter and angry at them and would rather see them suffer when an alternative remains, or this was part of the plan all along.

...or because the Rejoinings don't necessarily automatically yield ancients. We've seen this from the Source vs the First, where the life forms are more or less indistinguishable barring soul density, and very distant from an ancient. Bearing in mind that amongst the sundered are fragments of ancient souls which they very much would've wanted to restore, like Emet's old pal, Azem. Which makes arguing back from their end result and what would be required at the end of them to restore the ancients, to what would've happened before then when we're talking about vaguely specified "new lives", an exercise in futility. You're free to consider it as evidence of some prior plan but it's not very compelling because of all these confounding factors, IMO.

Meanwhile in Venat's moral system, it's fine to sunder the entire world because she deems it for the greater good.

There's the fact that Venat and her group objected to it pre-sundering. In fact, the new life would have to be of such quality to be offered up as sacrifice to Zodiark in order to revive the lost Ancients.

Yes, and why did they object to it pre-sundering? Because they thought the ancients were on a path to repeat their doom. The entire reasoning for surrendering the star over to the new lives ties into that.

And the latter part is a non sequitur. It would only need to provide sufficient aether for it, as it is aether Zodiark required. Again this is accounted for in the part of their plan where Zodiark stimulated the planet's natural vitality until it was bursting with new life - no doubt on account of his aspect of darkness. So it may be a case where the quantity here would do the trick.