The post credits scene teasing Pandaemonia may be Lahabrea.
I think this is all adequately explained in the story.
Venat couldn't risk bringing anyone in the convocation into the know about the Final Days, and without Hermes there probably wasn't much she could do about it. We were sent back precisely because it wasn't hopeful that it could be averted in their time, and we needed to solve it in our time.
Plus, as sundered beings, we were much more capable of dealing with Dynamis than the ancients were, too full of aether to perceive, use or combat it. Hermes has created a being the ancients couldn't defeat, only we could.
Even if she did have a plan and put it into motion once she had recruited her confidants, it's implied she would have failed simply because of that fact.
Overall I think Endwalker was done really well. Not as elegant or well-crafted as Shadowbringers, which had a very neat and focused plot with one amazing, really big pay off. Whereas Endwalker's strengths was in tying up four previous expansions worth of plotlines, which is not an easy thing to do, yet it pulled it off. It managed to be more fragmented than Stormblood, but it paid off every plot line really well for the most part.
My main disappointments would probably be:
Zenos, definitely the weak link of the story, seems forced and along for the ride. Probably would have worked better if he did take over Zodiark and he died on the moon.
I also think Anima and Hermes would have made much more interesting trials than the Zodiark and Hydaelyn that we got, but when you only have two mid-game trials to work with, it was inevitable that the more prominent characters would get them.
I suppose it may have been possible to split it into two entire expansions with a bit more world content, in order to give Zenos/Anima/Garlemald, and Amaurot/Elpis/Meteion much more focus and depth, but I'm not sure a two-year gap between them would really work that well.
Maybe Garlean centric expansion, with Anima and Zodiark and the mid-trials, and Zenos as the final trial, with another location in Garlemald (Corvis?) and two lunar zones in place of Elpis and Ultima Thule, then follow up in the patches with, trials for Hermes and Meteion in 6.1 and 6.3, with Elpis and Thule simply being additional smaller instanced zones. But that would leave 6.0 on a bit of a cliffhanger that wouldn't be resolved for nearly a year.
Either way they would likely have to play it as "Yay Zodiark is dead! the Ascian threat is finally over!" then 2-3 patches later it's "The final days are upon us", which would kinda mess up the Sharlayan and Thavnair sections.