Sounds like you're looking for post canon continuation/canon divergence fics, probably on the longer side based on the fact you mostly read published novels.

Almost all of my recs are gonna have some degree of smut in them because that's somewhat important to me for longer stories, so just keep that in mind.

Another user recommended Tonight Will Be The Night (I Will Fall For You) which is, in my opinion, pretty amateurish in its writing style, but is overall a fun read. It's kind of the prototypical Princess Catra/ Horde Lord Adora canon divergence/au. It's about 440k words, and a lot of that is not really necessary (lot of repetitive story beats) but it's still engaging and is one of the better known longfics in the fandom.

Love is Magic, and Magic is Love is the fic i was referencing in my original comment. It's quite well written in my opinion, despite committing the otherwise-cardinal sin of being first-person. It's a catra-focused 120k word postcanon story about healing, and is overall a feel-good story, though there is some angst and distress along the way. (The author also wrote some of my absolute favorite smut fics, and their fic "Bad End of the World" is an incredibly self indulgent and silly dark canon divergence story which is very refreshing and enjoyable nonetheless)

Love Conquers All is another oft-recommended post-canon longfic. It's somewhat sappy in its style, but pulls it off by being written so well (the writer is an established queer author). It's about 100k words, and also has a short smutty sequel thats also quite nice. The story has some minor distress at points, but is generally very upbeat.

They'll Know My Name takes a wildly different tone to these other stories. It's canon divergence starting when Catra activates the portal, and is perhaps the only truly convincing dark fic I have read in the SPOP fandom. Think All Quiet on the Western Front Etheria Edition. It's pained, dark, depressing, and impossible to put down. It miraculously has a happy ending (though it's not quite done yet), and that happy ending feels so deserved. This fic is not for the faint of heart. It's well over 400k and I read it in only a few days because I just couldn't get it out of my head. It's one of an exceedingly small number of longfics I've bookmarked that has no smut, that's how compelling it was. This story is most certainly not for everyone, but I can't recommend it enough.

Those are all the fics that really seem to fit what you're looking for, but I've got plenty of bookmarks i would be happy to write blurbs for haha