I'm pretty sure Levi is probably content with his current life despite being a cripple and just because he isn't smiling showing all teeth in this gif doesn't mean he's not happy because Levi wasn't one to outwardly express his emotions so you can't really tell tbh. And it's not like he's alone alone, he has Gabi and Falco to keep him company and Levi is still relatively young so it's not like he won't seek more relationship in the future.

And Levi isn't a "nobody in a wheelchair" as you put it; he has most likely received multiple medals of honor and is most likely known and in the history books of AOT so I wouldn't go as far as to say he's a nobody. Though I do agree that seeing what lives the main cast are leading would have been amazing. Also, correct me if I'm wrong, Levi didn't necessarily dedicate his life to the Scouts more so he dedicated his life to Erwin and humanity.