No, you are not entirely wrong about the whole point or central theme of the series from TG to RE. It's about the sense of life; essence of humanity; what makes a human, biology or its culture and society, mores and tradition? It's what everyone has been challenging and making Kaneki prove to the rest of Tokyo. Rize and Yamori's "lessons" support that of Eto and Arima's
Yeah, you're right!! And it's ironic because the four of them, that you mentioned, lived a pretty shitty life, Kaneki also has a shitty life, but he is different from others, well he is the only of the five that has friends and people who loves and supports him, maybe that is the reason why he keep fighting to live, and the others stayed halfway... ironic because of the fact that strong people who couldn't make it till the end, teached him how life really works (like.. their own philosophy or purpose in life didn't work for them) (i had a hard time expressing this thought but I hope you understand what I meant hahaha)...
Not sure about Kaneki having friends and people to support him. Growing up he only had Hide, comparing the others, Rize had Furuta somehow and Shachi, Eto had Noro and Tatara, Arima had Take and Hairu, Yamori had...I don't know, except him, I guess. So somewhat they are all at the same level. It's just that Kaneki have lived as a human who had no idea what ghouls are or what they truly go through, he's only seen them on news, up until the Rize incident. Humans have the tendency to think they are always more important than any other thing, that only humans struggle with life, and human struggles are the only ones worth caring about. Having Kaneki realize that ghouls suffer even more, they go through the same or even worse torture, is Kaneki learning about true empathy, since he has suffered pretty badly, from childhood to present. Well, that's hoping Kaneki's life is enough to make others emphatize with ghouls. And I agree with your irony. Damn. Ishida's good.
Rize once told Kaneki "choosing both, losing both", a long time ago Kaneki made a choice, he choose ghouls over humans, and now Touka over everything and everyone... and that is a fact. Of course this isn't a romantic manga, i know that, but if we reread everything since chapter 1 of TK to this last chapter of TK re, we can understand what is this about, it is life and the things we choose, the things we live for..."the sense of life" (of course this is fiction and things are completely different to real life)(and of course, i am not establishing this a the main goal of the manga, this are just part of my thoughts). I wonder who will be able to stop this Kaneki that has gone mad, but i think that Hide's coming back is the answer to that thought...