Demon Slayer: I Cosplayed Doma, Transmigrated, and Became the Ice Hashira

Don't let this one pile up in your reading list, or you'll miss out. [Cosplay transmigration, fun-loving protagonist, kicking over the dinner table, unconventional, no disguise, no System, no romance, no female lead, everyone is just friends, starting from scratch] Douma transmigrates into Douma. He adapts just fine, but why is he a Hashira instead of a demon? Well, fine, he's a Hashira, but why did he transmigrate to the Heian era!? Now that he's transmigrated, there's not much Douma can do except take things one step at a time. But can someone explain what it means that, because he earned so much merit in the life before his last one, he has to live until the eighty-ninth century in this life??? Douma had been planning to become a demon and follow Muzan, but now he doesn't know whether to laugh or cry. The good news: he can live a long time without becoming a demon. The bad news: he has to live far too long. Since killing himself doesn't work, he has no choice but to take things one step at a time. When he meets the young Muzan, he teases him and keeps him from killing indiscriminately, earning merit for himself in the process... Then what do you mean there's a new Demon King killing people left and right, and he's using Muzan's name? When he meets the young Tokito brothers, he plays around with them, and later joins the Demon Slayer Corps when they grow up and enlist... Then what do you mean Muzan killed him, and the entire Demon Slayer Corps wants revenge for him? When he meets young Hakuji, Hakuji asks him to turn him into a demon, and Douma regrets not being there for his childhood. Then what do you mean Hakuji doesn't want to become a demon anymore, and wants to live as an ordinary human and spend his life in love with Koyuki? When he meets little Douma, he just says hello, and they immediately click. Then what do you mean little Douma doesn't want to be a cult leader and wants him to take over? It's fun, but why does everything have to happen all at once? (Couple: Hakuji and Koyuki)