Kaigaku Munches on Peaches Online

My name is Ku Yue. I'm a transmigrator with no great ambitions, content to coast through life and wait for the end. My junior disciple, Agatsuma Zenitsu, is a genius, a hard-hearted man [boy] who hates women and supposedly has never cried. I'm an heir living off the old master's savings [peaches]. More than once, I've sighed over how the Agatsuma family really has no rotten eggs. My junior's two other friends are Kamado Tanjiro, a cold-faced demon hunter with no emotions, and Hashibira Inosuke, so refined and particular it's almost ridiculous. The three of them together make me marvel at how much they resemble a Jump manga trio. Then one day, while sneaking off to eat peaches behind my junior's back again, I stepped into a new world with one foot- I actually saw my junior crying his eyes out and begging a demon not to eat him?! I was left with a big, confused question mark on my face. ?! Which demon made such a fake illusion? How dare you make me look at something so horrifying? I'll kill you, okay?! Please note: 1. This is not told in first person. The transmigrated version of Ku Yue has never watched Demon Slayer, but he's great at making references. 2. Before transmigrating, Ku Yue lived in an inverted Demon Slayer world. I'm not whitewashing that scumbag of a senior brother. 3. Thanks for asking. There are no secondary couples, except possibly Obanai and Mitsuri. Honestly, I can't understand breaking up that pairing without a good reason. 4. Sorry, I'm such a bad writer that only the blurb is readable. Theme: The senior brother is very busy (busy eating peaches)