Dark Elves

Okay, so serious question: is the backstory of the Dark Elves part of MCU canon?

The new film "Eternals" stated that the Celestials were the only thing predating the Big Bang, ruling the primordial cosmos with an iron fist. But "Thor: The Dark World" says the same of Dark Elves. This completely retcons the idea of the Ginnungagap. I read this confusing article on MCU Wiki that said the Dark Elves surfaced into the light after the Big Bang, and that they went to Svartalfheim to sustain themselves, and Malekith wants darkness because apparently it will somehow make him all-powerful, it's all very confusing. How can Malekith be "unchallenged" by beings such as Arishem the Judge and He Who Remains when he can't even beat Frigga? Didn't it say both of those godlike entities were in charge, their ability to skewer Frigga in a heartbeat notwithstanding? The more I think about this, the less sense it makes.

The catch is, it was only covered in an Asgardian book Thor mentioned Frigga read to him as a boy, titled "Book of Yggdrasil". That may have just been a story in-universe. The events of "Eternals" contradicts it completely, so is the Dark Elves' origin still MCU canon?

If Malekith and the Dark Elves had no trouble invading Asgard, why did Thanos wait until after Ragnarok to steal the Tesseract?

Was he worried that Asgard's forces may have been too much for him and his armies?

Something I've noticed is that the villains (apart from Loki) in the Thor movies haven't been well written. In Thor 1 we have the First Giants. What do they Want? unclear. Why do they want to invade Earth? unclear. Same thing happened but much worse with the dark elves. Plunge the universe into darkness? what does that even mean???? But Then when Thor Rangarok revealed asgard was an evil empire I realized this could provide explanation for the earlier villains, especially the Frost Giants, who were likely murdered and oppressed by the Asgardians, so a vengeful Laufey decided he wanted revenge. Now if they had actually explained this in the first movie it would have actually made sense. I hope the fourth Thor movie will reveal that Asgard is improving its foreign relations with them