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

[Lauront]

and the other being that she felt strongly opposed to the third sacrifice, but these don't quite mesh together well under scrutiny. If Venat intended to Sunder the star for the purposes of Dynamis, then why would she feel the need to wait until two sacrifices of her people had already taken place? Especially if the idea of a third is bad enough in her eyes to serve as a secondary motivator altogether.

Although I think you are probably getting at what I am about to say, just for clarity's sake: it's less that she's opposed to the sacrifices as such, and more that she's concerned that if they were to complete them and restore their world and return to things as they were, they'd reach the fate of the Plenty, i.e. the third of the Dead Ends... about which she hears a couple of lines in Meteion's report, and... that's it. Enough to confirm her fears/beliefs. Nonetheless, even with this qualification in mind, it does not resolve the tension you mentioned, in the sense that even if her people agreed to her "accept suffering" mantra, if dynamis is an overarching concern, wouldn't she want to sunder them anyway?

So, agreed, I don't think either of these two points stand on particularly strong footing (could they not figure out a way to manipulate dynamis, even if indirectly, and make changes if their path would potentially doom them/their star?) and I believe that is why they threw in the time loop, which can be interpreted as her favouring the WoL's timeline (the possibility of AUs in this setting throws a wrench into this but how well that is understood by the characters in question is another matter.) Yoshi's Q&A answers does offer her trying to preserve the timeline as a possible motive, and it is by design she tries to spare Emet, which is consistent with such an interpretation. All this complexity and opacity around her motives is there because they're not very good if scrutinised, so we get the final Yoshi line of defence "She's an ancient, huh, doing ancient things." A position I can only arrive at by completely ignoring 99% of what they showed us about them elsewhere.

Anyway, agree with both you and Cutes in how morally fraught this ends up being. It really needed more than the tack-on Omega quest dialogue options given the very strongly positive leaning narrative she benefits from in EW.