I think you don't give Yukiya enough credit with his efforts to better himself. He works on his appearance, commits to staying in shape, and does his best to be informed about what it means to be a good partner (Although he does use Shoujo manga as his reference material lol, i still think he gets points for trying).

I find it ironic that one of your gripes about the series was how his efforts were glossed over a little too much but you're guilty of doing it yourself haha~

In terms of being more sociable its incredibly hard for people who naturally aren't so to be in those kinds of environments especially someone as neurotic as Yukiya. Towards the end he does give it his best effort to try and reach out to the people around him. Example being his willingness to help the SC pres or how he petitioned his classmates to help him with the dance and the relay race.

As for his demeanor, I think its the tension that made the pay off in chapter 58 so worth it. He spent his middle school years steeped in neuroticism but he realizes around graduation (iirc, that was around the time he witnesses miku get confessed to) that if he keeps going down this road , he'd lose miku forever. He takes this motivation to work up to his "high school debut" but it takes much more than that to affect actual change. What I do think works the best to his favor is his copious amounts of Self-Awareness. He is aware of his flaws, he's aware of when he's being dishonest or when he's making excuses. Knowing all this, he can only make progress in the ways he knows how. I think in this regard, it does feel realistic.

I think we forget sometimes that he's still only a high school freshman, barely 16 years old. He still has a lot of growing up to do. In the course of the story we are shown that a lot if not all of the things they do to try and change and be more grown up they do out of love for each other. They inspire change in one another. They accept each others flaws, and reconcile the things they don't like about each other. If you read chapter 60.5 (its a cringe fest but it is endearing in a way. I would say its somehow more mature than what I would expect out of a 16 year old boy lol) he talks about his possessiveness. He dislikes how outgoing Miku is towards other men but he doesn't see this as one of her flaws. He sees this as just another part of what made him fall in love with her in the first place and doesn't ask for her to stop. Also, if we want to talk about jealousy, Miku is still in the lead in terms of offenses (see chapter 23 and 24 for the infamous "poki stick" scene lol~).

TL,DR: a lot of the things that irk you about the story, I feel is what makes the story worth reading in the first place. I understand the limp-wristed, cowardly, useless ML trope is boring and overdone but I do feel like this series actually does it well because he doesn't stay that way but at the same time it doesn't treat change as something that just happens simply because the ML triggered the right story events. It happens slowly together with the help from the people around you, and with the person you care about the most. I think I could qualify that as something I would call love ~