Well, start by setting a clear goal for the training. Make the environment and challenges realistic. Also, show the character's growth and emotions during the training.
One great 'Muichiro x Reader' fanfiction is 'A Meeting in the Mist'. It beautifully captures Muichiro's somewhat aloof nature while gradually developing a sweet relationship with the reader. The descriptive language used really makes you feel like you're in the Demon Slayer world with them.
One common theme is the deep emotional bond between them. Given their relationship in the original work, fanfictions often explore how they support and care for each other on a more profound level. Another theme could be adventure, where they go on quests together and their relationship develops during those experiences.
The relationship development between Muichiro and the reader has to be believable. It can't be too rushed. A slow - burn, where they gradually get to know each other, understand each other's quirks, and form a deep connection over time is much more appealing. Also, the setting should be in line with the Demon Slayer universe. Whether it's in the Demon Slayer Corps headquarters or on a mission, the details of the world should be well - described.
Muichiro's death in the manga occurs during a particularly intense confrontation. His injuries accumulate, and he ultimately succumbs despite his valiant efforts.
I recommend you to read the novel "Martial Arts Fanatic." The main character of this novel was also skilled in martial arts, but there was no problem with showing mercy. When he was challenged, he would always give it his all without hesitation and show no mercy. The growth of this protagonist is also very inspirational. I believe you will like it. I hope you like this fairy's recommendation. Muah ~😗