Honestly, I've spent way too many hours on this. If you're looking for Namgyu x reader fics, Archive of Our Own is the undisputed king. The tagging system makes it stupidly easy to find exactly what you want—fluff, angst, smut, you name it. You can filter for completed works, word count, and even exclude tags you hate. The quality control is generally higher than on some other sites, probably because writers there tend to be a bit more meticulous about formatting and warnings.
That said, don't sleep on Asianfanfics. It's a mess to navigate and the search function is basically a random number generator, but there's a specific vibe there you don't always get on AO3. The stories feel... grittier? More immediate? I found this one fic there years ago where the reader was a convenience store clerk and Namgyu was a regular customer, and it had this slow, melancholic feel AO3 writers often over-polish away. You just have to be willing to dig through a lot of poorly tagged, abandoned drafts.
My final piece of advice: Twitter threads. Sounds weird, but some of the most inventive, off-the-cuff Namgyu content lives in quote-retweet threads. It's ephemeral and you'll never find it again, but in the moment, it hits different.