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

[Absimiliard]

Again where are the direct sources of her actually letting her own followers be out of the loop for the summoning. Wouldnt the Ancients already know that fact when the same wanted to consume souls to summon Zodiark? Also the fact that she even had a group of her own kinda goes against the idea that she never told anybody the truth of what happened.

In-game text, developer commentary, and peripherals cover these things. One such instance of an interaction between Venat and her followers, which is to have taken place immediately prior to the Hydaelyn summoning, has one of them talking about the next time they meet. It's made even more apparent in the JP text, wherein the conversation has even less of a "this isn't our last goodbye" tone and leans much more into "we will definitely see you again" tone based on the selected wording.

Incidentally, Zodiark at no point consumed the souls being used to summon him. In fact, one of the primal's directives was to safeguard those souls until they could be resurrected. Every soul sacrificed to Zodiark was still "alive" within the dark primal until the moment of its destruction. The Hydaelyn summoning, on the other hand, completely used the souls up. We're given to understand the limited number of sacrifices available to Venat at the time made this an inevitability - if not at the moment of summoning, then definitely when she used her newfound power to its fullest to sunder the world. All that remains within Hydaelyn by the time she tests the WoL is her own soul and enough stockpiled aether for one last confrontation at the height of her power, after which her own soul's aether is left depleted. Using up all the gas in the tank means your soul gets destroyed. You don't get an afterlife, and you don't get to reincarnate; you are unmade forever.