The nature of the Sundering has been spelled out in a Nier Reincarnation side story.

[Lurina]

My complaint here was more pointed at Shadowbringers as the initial instigator than Endwalker, but I do agree. I've heard that from around Shadowbringers on (which feasibly could've also influenced Stormblood and its patches) they wanted to write a story with no clear 'evil', where nobody is doing things for no reason beyond malice, because they don't want to put a story like that out into the world. And that's fine... but when we're talking about a setting where the main ongoing villains were omnicidal ghost wizards and a mashup of the Romans and the Nazis, trying to turn that setting into a story with 'no clear villains' apparently means finding pathos that justifies extremely horrible crimes. It doesn't feel right to compel forgiveness and liking for Emet-Selch, given his very tangible actions.

I've mentioned before that I swallowed Endwalker's attempts to do so much better, I think because the crimes of Venat, Fandaniel and Meteion are so much more fantastical and less relatable to real-world events. Venat feels like a figure in a creation myth, Fandaniel feels like... well, a JRPG villain, and Meteion feels like a figure in a philosophical thought exercise like Laplace's Demon. That's actually a good angle for villains that 'aren't clearly evil', because their crimes become abstract and conceptual rather than things with real-world analogues. To me, the Sundering is like the flood from the story of Noah, it's too impossibly enormous to be declared much of anything.

I don't really share your objections towards humanizing the Ascians, or even per-se the Garleans, though the latter was definitely a more delicate needle to thread that they botched quite badly, especially in Werlyt and the Gaius plotline. I enjoyed the central conflict in ShB a lot because, even though you developed an extremely strong sense of Emet's motives and could easily come to empathize with him, the narrative never expected you to understand the death he caused as justified or necessary. The coda on his arc only asked you to acknowledge him as a human being who was ultimately pursuing what he loved, while at the same time understanding the tremendous suffering it brought to others. It never glorified the harm he caused, just the love that motivated it. And you weren't expected to forgive him, just to understand him. Shadowbringers is a story about finding empathy and common humanity even in your worst enemies, while still fighting for what you believe is right.

...though admittedly, the seeds for undermining this careful tone were already being sown in the patch content, IMO.

Endwalker, in contrast, feels like it wants you to think of everything Emet and Venat did as just steps on the way to creating the good end. This is most embodied in the flower scene in Ultima Thule, where Emet is talking about how Venat manipulated him into playing his part like the whole scenario is some light-hearted joke. There is no weight given to the lives taken by any party, just sentimentality. It's gross, and worse, it's boring and empty.

Again, I don't really get where you're coming from with Venat or Hermes. Meteion gets a pass because she's literally not human, but Venat is portrayed as a human lady who broadly does human lady stuff until she whacks the planet with her sword. Garlemald aside, her actions feel no more or less grounded to me than the spooky void wizards smashing parallel worlds together.