Man, that's a specific pairing from 'Oshi no Ko,' isn't it? You've got good taste. Honestly, I think your best shot is going straight to Archive of Our Own and using the right tags. Search for 'Hoshino Ai Sarina Tendou' and 'Hoshino Aquamarine'—sometimes the character tags are under their real names. Filter by kudos or comments; that usually sorts the wheat from the chaff.
I've found some real gems there that explore their weird, co-dependent dynamic beyond the idol/manager thing. There's one called 'Ghost Light' that really gets into the psychological manipulation angle, which felt pretty true to the source material. Just be prepared to wade through a lot of... less polished stuff. The fandom's still growing, so quality can be hit or miss.
Sometimes, cross-posting happens on Fanfiction.net, but AO3's tagging system makes it way easier to find exactly the messed-up relationship studies you're probably looking for. I'd steer clear of general anime fanfic sites for something this niche.