Finished rereading that part last week, and it's kind of brutal but fitting, I think. By the end, Jiang Cheng is left pretty much alone, running Lotus Pier and raising Jin Ling, which is what he always said he was going to do. But you get the sense he's just going through the motions, you know? The guy lost his whole family and his brother twice, and even after they sort of reconcile, Wei Wuxian rides off with Lan Wangji. Jiang Cheng's ending is him standing on the docks watching them leave, again. He's got his sect, his status, but the narrative makes it painfully clear he's isolated in a way Wei Wuxian never will be. It's not a happy ending for him at all, but it's not like a redemption or a comeuppance either—it's just him, stuck with the consequences of every choice he and everyone else made.
What gets me is the clarity he gets in the extra chapters, realizing how much of his own misery was self-inflicted, that Wei Wuxian didn't actually abandon their bond. But the knowledge doesn't really free him. It just adds another layer of regret. He's not villainized by the end, but he's not exactly absolved either. The story leaves him in this permanent, bitter state of having everything he thought he wanted and nothing he actually needed.