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.

The thing is, Hachiman is a painfully relatable character for me. Not now me, but slightly younger me who thought the same way as Hachiman. It's a mindset that really does exist, it's the type of thing that pops up in smart teenagers who end up on the wrong side of people, the isolation grates on you, you grow cynical, and you put yourself on some pedestal to try to justify never getting to a better place. You become sure of yourself, entrenched in your own philosophy based on nothing specific besides being a bit jaded to the world around you because you don't fit in fully. Maybe it's all you, maybe the world would accept you if you tried just a smidge harder, but at this point you've built walls and it will take someone hella persistent to bring them down.

That's Hachiman, and I think it's also a lot of kids around his age. It certainly was me for a little while. Hachiman can be bland if you don't understand this though, if you don't fully get where he's coming from he may seem unjustified, and he may very well be unjustified, and I think that's sort of the point.

The show goes on to spend it's runtime presenting Hachiman with simple but not unrealistic situations and characters, and having them play off of him. This is used as a way to explore Hachiman, occasionally his philosophy, but more often to show his unfaltering resolve to himself, his ability to ignore or warp reality to keep his own misconceptions about the world in tact. The show is quick to deny most of his points, or at least to take away some of their credibility. Characters like Hayama and Yui exist mainly to do this, they fit their persontypes well and both of them treat Hachiman with far more genuine kindness than he'd like to admit.

I'm not saying that you have to, or even should like it. It targets a particular audience of people and does it brilliantly, but if you fall outside of that range it's probably going to be pretty disinteresting.