Shinsha is Phos' antithesis. But now, The World is their enemy. Shinsha is the epitome of "I cannot do anything". They take no risks, only do what is expected, and have no ethos beyond needing acceptance. They have no desire to become stronger, because that would threaten what they have, and while that's next-to-nothing, they have ownership over it, so it's priceless.
It's odd that this manga is about Buddhism when nobody seems to Want anything... isn't that supposed to be the end-goal of Buddhism?
Except Phos. They have desires. Thus they suffer. But that also makes them alive. I guess the manga asks a hard question: Would you rather feel alive and suffer immensely, or be sedated and numb, understanding nothing? But most of the gems don't actually make a concerted effort not-to-understand.
Why? Because Sensei inflicted Phos with understanding. He, the teacher, put Phos on the path that would make them suffer, but lead them to enlightenment. See this image? This is the first fucking chapter. Everything Sensei talks about has come true. See the final panel in this exchange? Sensei knows how this shit all goes down. Give someone the means to understand the world and they'll become a being-apart from most people. And that "I leave it to you"... he knows can't fix the current situation because he IS the situation.
So yes. It's horrible what's happened to Phos. But it's all part of the process. To be alive is to suffer. And damn does Phos feel alive.