There's this one site I almost never see mentioned in those big lists, but it's been a lifeline for me: Dreamwidth communities. It sounds old-school, but some of the best, most nuanced sickfic writers migrated there years ago to escape algorithm-driven platforms. You have to hunt for locked communities and get invited sometimes, which is a pain, but the quality is unmatched. They treat the tropes with real care, focusing on the emotional weight of vulnerability rather than just the fever symptoms.
AO3 is obviously the giant, and their tagging system is a godsend. You can really drill down into 'Hurt/Comfort', 'Sickfic', and then pair it with your specific ship. The trick is to exclude tags you don't want, because the popular ones get flooded with shorter, tropey pieces. I sort by kudos but then filter by word count if I want something substantial.
Tumblr is a weird mix. It's harder to search, but some writers post exclusive threads there or link to their AO3 works. Following specific sickfic blogs or tracking the #sickfic tag can yield some surprisingly tender one-shots you'd never find otherwise. The platform's visual nature means you sometimes get moodboards with the fics, which actually adds to the atmosphere.
Honestly, my most cherished finds have been through Discord servers dedicated to a particular pairing. Someone will drop a PDF or a Google Doc link of something they wrote years ago that never got posted anywhere public. It feels like finding buried treasure in your own backyard.