Does that not count as developement in your eyes?

She hung out with the boy she likes and the girl she looks up to for over a year. No, knowing how they act is not development to me, it's the absolute minimum effort acquired to keep a friendship going.

Also why do you feel like her only real reason to exist is to make the other two more interesting?

Because there is never any focus on her. It's always her and someone else.

Her way of thinking and her actions give a nice contrast to the other two main character, similiar to Hayato.

Hayato is a walking trope. The dude is the perfect guy. I wouldn't say that he's a good character and there good counter to anyone else in the show.

I have a feeling it's significantly easier to understand the characters if you are introverted yourself

I feel more that the show fails to execute what it tries to do by focusing too much on making a usual romcom, because Hyouka is a show very alike to SNAFU when we're talking MC's turning away from the world. But Hyouka gave the necessary insight in how the MC got to where he was, where as SNAFU never bothers and simply showcases that he is cynical, rather than have it be grounded in his past.

There were several parts where he was deeply affected by what he did.

I wouldn't define them as deeply, given that he got over them in a nights sleep every time. And if he didn't, then they should've shown that. They never showed me there was any lasting struggle about his actions though, so I don't feel like Hachiman is a character that made a memorable change.

Hachiman did grow. Not far, but that's understandable. Someone with such a worldview doesn't become completely social in several months. Breaking free from a delusion you have set for yourself is hard, even more so when that delusion the way you want to see the whole world around you.

But he doesn't have to become completely social. All I want is for him to make a meaningful change, but the show refuses to let him by always shifting the focus onto comedy for whatever reason. Everything that could be meaningful or interesting is off-set by the lack of character building he got at the start that invalidated his development (because who cares about developing a character that never was a well-rounded character in the first place?) and the absurd amount of screentime wasted on completely useless scenario's.