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

[Theodric]

We already know that her intention was to eliminate her own species and replace them with a different species, acknowledging just that when confronted by the Warrior of Light and Scions, as per the conversation with Y'shtola:

Y'shtola: Having seen mankind brought to the very precipice of extinction, you wished for us to develop a means to overcome despair.
Y'shtola: You believed we had the potential, and sundered all creation to see it fulfilled. To deliver us to that swirling maelstrom of dynamis in which our foe hides, and grant us the power to defeat her once and for all. Is this not true?
Hydaelyn: It is as thou sayest. 'Twas the trial to which I subjected mankind, and it hath led to untold bloodshed and suffering. There was no kindness nor justice in the tragedy I wrought.
Hydaelyn: When confronted with the almighty Zodiark, my only recourse was to rend Him and the world asunder, that His power be diminished for a time.
Hydaelyn: And so it came to pass. Now you, my chosen, have surpassed my expectations. Surpassed me.

So, yeah - she's explicit in regards to it being her goal, which Yoshi confirms in the previously and repeatedly linked Q&A in addition to her fears about her people eventually becoming like the Nibirun. Zodiark was needed to maintain Etheirys' shield and had to be Sundered because he was an obstacle to Venat's plan and while she could not fully control the scope, Zodiark's power was so great she ended up having to sunder all life in addition to him, but that's collateral damage as the Ancients are very clearly stated to be the main target.

Again genocide would be doing it because she is targeting due to race or ethnicity, or even people with specific aether density perhaps. But the sundering affected all aether density of everything. You can't even really invoke eugenics here because she didn't target human reproduction but aether levels.

You can because she is trying to bring about a new mankind she believes is better suited to a specific problem she has in mind so reproduction is irrelevant because she has magical means to effect it. There is, of course this as well:

Hermes: Harboring high concentrations of aether, we ancients cannot readily manipulate dynamis─nor be manipulated by it. Therefore, rather than our selves, the calamity affected our magicks.
Hermes: In contrast, having been sundered, the people of the future are composed of but a fraction of our aether.
Hermes: Thus are they susceptible to the influence of dynamis─and its transformative potential.

So it was very much a deliberate act and an intended consequence. The Sundering was always about replacing her own species with something different in an effort to bring about Meteion's defeat. A combination of action and inaction resulted in a heavy cost and it strikes me as rather strange to downplay and ignore that cost to try and frame it as something completely different.

Even in real life genocide is applied with regards to intention to harm, oppress etc. I'm sorry you feel how you do but Venat's intentions were oriented around long term survival and the preservation of the star. It's another disqualifier. This is why I've said since the beginning intention matters. She didn't reduce aether levels across the star for some nefarious reason.

There's no asterisk in any definition of it that says it doesn't count if the perpetrator thinks they're acting for the greater good. In itself, there are various forms of genocide both direct and indirect, soft and hard. It can be as simple as killing large swathes of people with direct murder or cutting off supply lines in order to cause them to starve. The method matters little as opposed to the intended result and we know that the intended result was the deliberate eradication and replacement of her own species.