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

[Theodric]

This has been alluded to for a while and isn’t something that they pulled out of nowhere.

I assume you can provide the appropriate sources, then.

Their souls were used up and converted to aether and what remains are the dregs rattling around.

Where was it stated that there were 'dregs'? Or are you simply assuming that they existed? How does that align with the previous declarations that their souls were entirely used up?

the foreshadowing that’s been there for over a year now, or are being pedantic.

Furthermore could you also explain, using the lore and not your own headcanon, how their essence will unravel, return to the star, and give rise to new life from mere leftover fragments that you are claiming have been foreshadowed to exist for over a year?

Didn't Yoshi-P mention something about Venat having completely consumed the souls of her disciples?

He did, here's the quote in question:

Q: Venat said that not even her soul would remain, but what does that mean? I’m really fond of her character and want to see her again.

A: Souls are also made of aether, and she had used so much aether that even her soul was gone. Zodiark absorbed those who were sacrificed and was still able to maintain their souls within, whereas Hydaelyn did not have the leeway to preserve the souls of the summoners, including that of Venat herself. This difference was because Venat’s group was a very small minority compared to the group that had summoned Zodiark based on the Convocation’s decision. Venat’s soul, which was the last to remain, was used up in the trial against the Warrior of Light and their allies.