I've read a surprising amount of Obanai x Mitsuri stuff, and honestly, the pairing itself is so specific and canon-compliant that the 'best' fics often aren't about them as a romantic duo. They're already established. The more interesting explorations I've found focus on the absence of romance in their backstories, or hypothetical alternate meetings.
There's a writer on AO3 who writes these tense, quiet fics set during Obanai's time in the corps before he meets her, where he overhears rumors about this unusually strong girl with pink hair joining. The longing is all in the anticipation and the distance. It's less about fluffy hand-holding and more about two isolated people slowly realizing their puzzle piece exists somewhere out there. Another good angle is post-final battle recovery fics, where they have to navigate a relationship without the immediate pressure of constant combat, which can feel very grounded and sweet.
I'd skip anything that's just a rehash of their canon dates. The magic is in the margins of their story, the things 'Demon Slayer' didn't have time to show.