So. I think there are 2 underlying themes, and
several motifs, which are really unable to be ignored. The first being that the world is innately cruel but that there is also beauty in it "this world is cruel but also very beautiful" which I think is the foundation for most of the other themes. Very early on you see that there's innate cruelty and despair that all the characters face, but they also recognize that there's goodness and beauty as well
(for Mikasa this is in her relationship with Eren, for
Ymir it's Historia, etc.). This isn't profound, but it's important. The second one is that the pursuit of the object of our material desires leads us inevitably to sin and regret. Few examples of this
"Everyone's gotta be drunk on something..." Erwins basement, Ymirs living for herself, Reiners desire for his family or glorification, Erens desire for freedom and revenge, same with Gabi. The list literally goes on. The common character arc is that somebody wants something, they engage with the cruelty of the world rather than the good and in this way they "sin," a term used often, and inevitably they regret their actions. The distinction between the good and bad characters made in the show is whether or not they recognize this and reorient how they approach finding meaning in life by resting their wills in the greater good.
The failing of AoT imo is that the "greater good" is just some nameless, formless, broader world. "We find meaning in our lives through those who follow." I wish it was God, but I don't really think it is. It also offers a critique of the dogmatic themes prevalent in most shonen, in favor of a much better orientation of the will to power. Often times you see the theme keep moving forward to realize your dreams, but AoT illustrates how to move forward at any cost only leads to evil and is an empty end (because we find meaning through the greater good). This doesn't mean "don't devote your hearts," it just means that will which isn't oriented towards the good is going to lead to disillusionment. I think that we see this in characters like Miche, or Eren. Miche died purporting that "as long as we have the will to fight we haven't lost" and his death was obviously stained with regret. Same goes with Eren.
There's actually a lot more to speak about tbh, but
I think these are the most glaring and indisputable themes of the show
I might make an actual post down the road, but I don't have the comic editor rn