I agree with you here.
And this is where you lose me. Well, not the last sentence; that is true. Misery builds character is an old adage that has some truth to it. It's less about what the suffering being good and what we take from the suffering. Yes, it's unavoidable, but wallowing in it doesn't do jack. The game, and many other stories, tell us that you have to 'grin and bare it because if you just suffer instead of just moving on, you're, well, dying in one form or another. We learn from suffering. We learn to have empathy, we learn to make better choices is that suffering was self-inflicted, and we learn what the red flags are on other people so we can avoid them. We learn our limits, and we learn how to push past them. We learn about ourselves. We grow from suffering just as much as we grow from good things.
We learn that the world is unfair, and we want to make it better for our progeny. They'll have a better life and better living conditions, hopefully, but they'll still face suffering; it'll just be a different kind. Suffering is unavoidable, and yes, it's not good. But good things can come from it.
I think we need to remember that the devs are Japanese, and this is very likely a reflection of their beliefs. Nor is Buddhism the only faith that does this. "God moves in mysterious ways" or "It's all part of God's plan" are very Christian, or at least Catholic.
However, I do think you misunderstood what's trying to be said. In no way did I see the Scions say, 'let's accept this' when facing the Endwalker. They went to meet that threat, and have, throughout the game's expansions, went kicking and screaming to stop injustices. At no point were they passive.
Endwalker is not about passively accepting suffering. It's about accepting that it happened and moving on with life to be better. Isn't that what the scions are doing now? Aren't the twins in Garelmald right now trying to help Jullus rebuild their home? Aren't we trying to help the Void now? In the game right now, with the lessons we learned from Endwalker and all that came before, we are still doing our best to right wrongs and triumph over evil.
Because the reason Venat sundered the Ancients was not to force them to accept their suffering. It was to stop them from wallowing in it, and inflicting suffering on others. Let's remember that when the ancients sacrificed half and half again to stop the Final Days and begin to heal the world, they were planning on sacrificing the new life that sprung up afterward to bring back what they had lost. Inflicting more suffering because they wanted to stop their own. And that's unhealthy.