I genuinely think the only Truly satisfying conclusion of taira's personal growth for me is if he gains back the weight after high school and I'm not kidding!!! More yapping below.
Like.... He loses the weight thinking it will fix everything, which is understandable because he's a teenager who's bullied for being fat. And then he finds that not everything is "fixed", because he's the same on the inside and he's still awkward and negative and socially clumsy (almost certainly Because he's spent his formative years being bullied) and he needs to work on his self esteem and confidence and believing his friends like him.
Instead of treating this like "he fixed the outside, now he needs to fix the inside to match" (fatphobic), I rly think the answer is needs to be that the outside was never the problem, just the dickheads he was surrounded by (and what do u know, there's a parallel between him and azuma!!). We've never seen the current gang exclude morimo for his weight, for example - I think all signs point to "if taira had looked the same as he did in middle school, but grown his self confidence enough to socialise, the gang would have still been his friends".
Also, the fact that it was clearly a concerted effort to be a "new him" entering high school, and something he did very quickly for how skinny he now is (like over a summer?) says to me that the way he looks in the main timeline of polar opposites is not how his body naturally sits! and something he probably needs to maintain purposefully. I think as he grows up and stops being a teenager, it's very likely that he would gain weight again. And I think, after growing on the inside, he might find that that was actually not a huge deal.
I'm saying all this as someone who was a big/chunky kid, was reassured by adults that I would lose the "puppy fat" when I grew up, had a skinny-ish couple of years in my teens, and now in my mid twenties am the fattest I've ever been. And it's fine. My 16 year old self was absolutely terrified of looking how I do now, but it's literally just fine. People treat gaining weight like the end of the world and of course there are social impacts and people can be assholes, but it genuinely isn't this life ruining thing that I thought it would be when I was a kid! You can just be fat, and be happy, and it feels kind of strange to think about how scary that idea used to be to me.
I would love to see tairazuma in this context too, though I'm waiting to see how their relationship develops to form Thoughts.
Anyway. Fat taira summer. I haven't finished the series yet so if this conflicts with anything.. idk who cares ʕ ꈍᴥꈍʔ yay (◕ᴗ◕✿)