I'm equally displeased with the author, but I'm curious whether their Japanese fans are 100% in sync with the MC or are sending angry tweets to the author. All the cringy and stupid tropes that keep returning in manga and anime do so because they're common in Japan or because they're easy to write. And we all know drama clearly is not easy to write as this manga is going to shit whenever it attempts anything serious that is non-romantic. Anyone in the know?
I personally think it's a combination of all you said, combined with the fact that this is written by men, for men. On top of that, these mangakas seem to have very little real life experience to draw from, so they end up using what they know: stuff from other mangas, again written by men, for men. It's all they know, and the cycle repeats.
This also happens in manga written for women, by women, btw. They sometimes promote toxic stereotypes as well. I'd say the difference is that the market isn't as saturated on that end, and it's easier to find better written drama, I think.
In small-scale mangas like this one, I don't think the mangakas have a lot of input from others, they don't even seem to plan ahead. I often get a feeling that they just make it up as they go.
As you said, if they stick to something easy, they can get by, but when they introduce drama, which is much more difficult to write well, as it requires nuance, empathy, and above all, good storytelling, it all goes to shit.
Still, I don't live in Japan, so as an outsider looking in, I'm just as curious as you to know if there's more to it.