I disagree with the interpretation of the people of the plenty being "horrified" or "suffering" in any way. The emotional tones you say you see, I don't really read it like that. To me, it's more akin to simply being bored with life and happy to experience an alternative, not a horrible, despair-inducing existensial crisis.

It only looks like "despair" to an outsider because surely you would only embrace death because you are suffering in life, but I think it makes The Plenty more interesting and more alien if they willingly embraced thier deaths from a place of contentment and bliss, rather than outright despair.

And by "Meteion is using the races in Ultima Thule as arguments", I mean that she is presenting them in a certain order and certain perspective in order to make a point - which is something along the lines of "look at all these societies who died in suffering and despair after trying thier best, and then look at the Plenty, who died blissfully after acheiving perfection, wouldn't it be easier and better to just skip to the part where we all die?"

It's entirely possible that there were members of The Plenty's society that disagreed, or didn't want to die. But that doesn't really matter for the purposes that they are being used by Meteion for, which is to try to convince the WoL and the Scions that death is a wonderful and pleasant alternative to the suffering inherent in life.