I'm actually going to buck the trend and say the best Kurt x Ram stories aren't on the giant, generic platforms at all. For a pairing from 'Glee', which has such a specific, nostalgic early-2010s vibe, the real gold is on the older, smaller archives that were active when the fandom was hot.
AO3 obviously has a massive collection, and their tagging system is a lifesaver for filtering, but I find the writing there can skew towards very polished, often very trope-heavy stuff. Sometimes I miss the raw, messy, intensely emotional one-shots people used to just slap up on LiveJournal or their personal fanfiction.net accounts. You get a different kind of passion there, less curated.
For Kurt and Ram specifically, I'd honestly recommend digging through Tumblr tags from like 2011-2014. A lot of writers posted drabbles and headcanons directly to their blogs, and some of the most charming, in-character snippets for this slightly rare pair are buried there. It's a bit of a treasure hunt, but stumbling across a forgotten four-year-old post that perfectly captures their dynamic from a rival to lovers angle feels more rewarding than just browsing the latest kudos list.