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

[Lauront]

The ancients' souls were not consumed by the Final Days because of how dense their aether was, hence they were available to use to summon Zodiark and create a buffer (albeit as a last resort because they didn't know what was causing the crisis or what led to it spreading so rapidly), adequate to last at least 12k years even in sundered form, and also to revitalise the star with some more sacrifice. In a situation where souls are consumed wholesale, that just wouldn't be possible.

As to methods hinted at? Ktisis Hyperboreia allows for aetheric suppression, for one; uses of this could potentially be adapted and extended to allow them to more readily hone the ability to interact with dynamis. For another, use of better constructed familiars than Meteion. For yet another, selective sundering of some ancients rather than sundering the entire planet, and preparation of them to face her. It only took a few sundered to hunt down Meteion after she'd guzzled several stars' worth of despair. In addition, Elidibus in SoS is able to manipulate the power provided to him through prayer from summoned WoL fragments to fuel his LBs and shield himself from them, and we know there is an association between prayer and dynamis; primals are constructs of both prayer (a lot of which went into Zodiark) and aether and it is possible to turn into them and adopt some of their traits. These and other things they could've come up with in the span of 12k years or less were all potential workarounds...