I'd disagree on a lot of your points about the side characters, they're certainly not super deep, but I think the show is sharply written in the sense that they're pretty clearly drawn from real life and not random archetypes or stereotypes, for the most part. Which was kind of important for what the series was trying to do, or at least what it seemed like it was trying to do to me.

Hayama was pretty much a perfect boy who couldn't do anything wrong. I've never met anyone like him in my life. I've met plenty of good people, but all of them also had their shortcomings, which Hayama does not. The teacher isn't even close to being drawn from real life, neither is Yukino's sister - they're pretty much anime idols in the sense that most of their reason to be in the show is to please the otaku's who like older women. Then there are the chuunibyou and trap, and I can't take those seriously either.

Who comes closest to a real character is Sagami. Her desire to stand out and be appreciated combined with her lack of knowledge of hwo to act in that situation and devotion to putting in the effort were the first things to make a character feel like an actual person. But she gets swiped aside to get crushed by Hachiman to give him 'development'.

The thing is ... to bring them down.

I never really attacked the way Hachiman thinks I believe (or at least I never intended). What bothers me is the complete lack of effort the show puts into making Hachiman a good character. He just starts of as a cynical pessimist, and originally the idea is very good. But he only ever gets about a minute or five of background story in total, and that isn't enough to make me feel like he has a reason to be like this. And that's a big part of making a character like this work for me. I want to know what happened to him to make him into who he is right now. If not by showing me any background then by having him rationalize why he ended up like this. "Girl rejected me so I decided talking to anyone or believing in anything wasn't worth it anymore" doesn't really cut it for me.

That's my gripe with Hachiman. Yukino has a much better explained reason to be the way she is, and that only took the show about 5 lines to get across. Which is why it frustrates me that so much effort is put into making me care for Hachiman, when I never got a reason to and all I wanted was more development for Yukino, which there hardly was because of him sucking up all the screentime.