It's not just the events at Elpis that establish Meteion's character either. It's everything building up to this from all of the stories, and as the cause of the "Final Days" Meteion being the final boss is the last piece of the puzzle that explains pretty much everything.
Why are the ascians going around teaching everyone to summon primals? To bring back their god, Zodiark, which we later find out is done by causing calamities.
Why do calamities bring them closer to their goal? Because the world was broken into pieces and the calamities are part of putting it back together, which will put Zodiark back together.
Why was the world broken into pieces? Hydaelyn was summoned to do it, because Zodiark was a cruel god who demanded sacrifices to keep running.
Then where did Zodiark come from? He was created to be a deified embodiment of the planet so that it could stop a total global apocalypse by stabilizing the whole thing.
Then what caused the apocalypse? The Meteia sent out by Hermes to find life on other planets who found only death. Hydaelyn also knew about this and breaking up the world and keeping Zodiark around but sealed up was all part of combating this from the beginning.
The idea this is even remotely similar to Necron or Zeromus is laughable, because the whole story doesn't make sense without something in that place to cause it all.
Edit: Also, Fandaniel was barely any more fleshed out than Meteion was. He was only added in 5.4, and until the same scenes in Elpis and his flashback to being Amon he was just a generic crazy guy who wanted everyone to die. Additionally, he has his own purpose to serve being a mirror to our character (he identifies more as Amon after being pieced back together by the other ascians and not as Hermes, just as we don't see ourselves as Azem) that would not work as well if he somehow was the cause of the Final Days himself, not to mention the question of how they would make that even possibly work.