Oh man, that's a pairing I never expected to get so deep into, but here we are. Honestly, for that specific ship, you've gotta look beyond the biggest platforms. Archive of Our Own is the obvious starting point, obviously, because the tagging system means you can filter right down to 'Tokito Muichiro & Shinazugawa Genya' and find those gems among the general 'Demon Slayer' fandom. But the real treasure trove I found was on smaller, character-specific Tumblr blogs. There are a few writers who are absolutely devoted to them, and they cross-post snippets and drabbles there that never make it to the big archives. The tone is different, too—more experimental, sometimes just pure character studies without a big plot. I've stumbled on some breathtakingly soft, post-canon healing fics in those corners that reframe their entire dynamic.
That said, I've seen a weirdly high concentration of really good, novel-length stuff for them on Quotev, of all places. It's an older platform that doesn't get talked about much anymore, but the 'Kimetsu no Yaiba' section still has active authors who write incredibly detailed AUs. I read one last month that was a full-on fantasy royalty setting for Genya and Muichiro that was shockingly well-plotted. Wattpad can be a mess to navigate, but if you use the right combination of tags like '#genmui' and '#shinazugawagenya', you can find some heartfelt shorter stories, though the quality is more of a mixed bag. My advice is to not rely on one place; your best experience will come from checking AO3 for the polished, completed works, then diving into Tumblr and Quotev for the niche, passionate, and often more raw interpretations.