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

[Lauront]

I think we just haven't had a chance to see a lot of beings that are associated with Darkness. Maybe they all look like that and it's only our own in real life perceptions coming into play. To the people on the First, it's the angelic beings who are the true demonic personas.

In all honesty, I thought Zodiark's form was fine. They used SHB to hammer the point that simply looking like an angel doesn't imply one is benevolent, with Innocence as an extreme of this. Plus, some of the sin eaters and lightwardens themselves looked rather demonic. Zodiark by contrast has the large horns and menacing appearance, but he is a primal of salvation and also an Atlas like figure, protecting the star from corruption. I suspect Zodiark's appearance is simply a consequence of him being a primal of darkness, although given that the statues of him predate his summoning, it may have some mythological significance to the ancients as well. As in, he bears the guise of some manner of protector deity. Horned gods exist as a thing separately to demons.

Where it grated on my nerves was constantly Hydaelyn using the terms light and dark as interchangeable with good and evil (or associated concepts, like hope and despair), when they spent an entire expansion unravelling this association. It just sits weirdly. The fixation with light at this point is just dull to me.

Again, it wasn't a gamble when you know the outcome. She knew Sundering the world would put a temporary stop to the Final Days.

Hm, where did she know that from? Zodiark was what was preventing a repeat.

She heard our tales of stopping numerous world-destroying threats, she heard of us defeating Emet-Selch, who bare in mind is considered the most gifted in Aetheric manipulation out of all Ancients

Yes, coupled with seven other summoned champions, the Scions and a ton of light which the story attributed the MC being able to control due to bearing the Blessing of Light and which took a further soul shard rejoining to avoid bursting at the seams. The sundering aetherically reduced life 9 times in excess of the total amount of soul shards the WoL had rejoined. All that, plus the aid of the scions to eventually put a hole through him. And again, this is one singular individual in possession of a particularly potent ancient soul, with a lot of plot contrivances enabling said outcome. Rather exceptional compared to other sundered beings.

What she didn't know is what would happen when we returned from the past, and even then she put a plan in place in the event we failed.

An utterly futile one, because what does fleeing the star accomplish against a foe like Meteion?

She didn't know if we would be able to defeat Meteion, as her knowledge of the future ended when we returned to the present, so she tested us. If we failed, we were to flee Etherys. If we passed, then we know what happens.

This part right here is why people refer to it as a gamble, i.e. sundering the world to commit to the original timeline on the basis of an outcome she'd still need to test.