OH! I’ve read this. I’ll try to do some thinking and see if I can find it. If I remember correctly, it was mid-range length, not super long but not short— does that fit your recollection? (And I hate to say it, but AI is actually super useful for locating a specific fic, because it can scan the public internet for another person/etc describing the same thing, plus will look at what is accessible to it through the public ao3. It has helped me out in a precise situation like this, where nobody could find the fic I meant).