Demon Slayer: There's a Demon Inside Me!

[Demon Slayer + coming of age + everyone survives + canon couples stay together + no transmigration + slow-paced plot]

From the start, Himuro Hisanobu encounters Muzan and discovers a demon living inside him. The demon isn't a stranger, but his grandfather. Under his grandfather's guidance, he takes up a Nichirin Blade and joins the fight against demons.

At first, afraid that the Demon Slayer Corps will discover his grandfather's presence, he avoids contact with them as much as possible. Later, the demons he encounters mistake him for one of their own and try to recruit him to serve Muzan Kibutsuji.

Once Himuro Hisanobu is sure the Demon Slayer Corps hasn't discovered his grandfather, he chooses to join them.

After becoming the Ice Hashira, he encounters Kanae Kocho fighting Doma on his way back to the Demon Slayer Corps Headquarters. He joins the battle and learns the truth about his parents' deaths.

During the Mugen Train incident, he is carrying out a mission nearby when he hears that Kyojuro Rengoku has encountered Upper Rank Three and goes to help.

At the Entertainment District, he once again crosses paths with Tengen Uzui, who is fighting Upper Rank Six.

In the final battle, he joins forces with Shinobu Kocho to avenge his parents, then goes to support the other Hashira. In the end, they really do succeed in creating an age without demons.

(PS: The protagonist is the tenth Hashira and has his own character arc. He has no romantic pairing. He won't have much contact with the Demon Slayer Corps early on, and most of the original storyline won't change. Every character will appear. The author personally wants to give the Hashira a perfect ending, so this is a forced happy ending and everyone survives. Apologies for the out-of-character moments, apologies for the writing, and apologies for everything else you can think of!)