This manga went off the rails and accelerated up to the stratosphere in both story and art style right after the whole granny arc ended.
Even before that if you ask me, but going full supernatural wasn't that bad, quite unexpected given how late to the game it was, but still, it was in the realm of understandable.
The same happened in "Kotoshi no Mitsuishi-san wa Dokoka Okashil", at some point everything went off the rails and simply stayed there, but since it was pretty early on, it didn't feel as jarring as it feels here.
Not sure about the production timeline of both mangas, which one came first or after, but the fact that both share that aspect makes me think the mangaka went clinically insane around the same point in time, but since the manga was still selling nobody decided to swing the axe. lol