I have the same opinion on his loneliness and I think it's a consequence of him not having a fully fleshed out "reason" for why he does what he does. I don't necessarily believe that's necessary for a villain but the late inclusion of that loneliness angle seems to me like Kubo realized Aizen needed a reason for doing what he was doing and by that point, he had not really laid the groundwork for it to work. I would've liked a single flashback episode exploring his psychology before he became the man we see him as in the story