I just found a good one, imo. It’s still in-progress, but it’s really good
Fate Deferred by catie_writes_things on ao3
I’m not sure it’s a…. “happy” fic…. but Zuko and Katara are happy together, so it fits that requirement, at least!
I feel like I know more, but I’ve read so many fics I’d have to sit down and re-familiarize myself with a ton of them again before reccing.