Fictional Fans

Sort of a counterexample - I remember reading a published book some years back and thinking it was so screamingly obvious that the author came from a fanfic background, because throughout the book I was just like "look, you're writing origific, you can't rely on your readers to already know your characters were MFEO, you have to actually show us how they fit together rather than going from zero to sixty within two pages". So I think that's a factor, too, although it's much harder to pin down than the ones you mention - how much of the character interactions and relationships rely on pre-existing canon or fanon?

Also, so often people recommend short fic as "don't have to know canon", and while this is technically often true, in practice I find these to be the least compelling recs because often short fic in fandoms I don't know don't give me any reason as to why I should care about the characters and the situations depicted in the fic. So generally speaking longer fic I think is better.

And also, based on my own experiences? If you're in the fandom yourself, figuring out what's don't-need-to-know-canon and what's not is SO damn hard. The best don't-need-to-know-canon recs start with "I don't know the first thing about this fandom but I loved this fic."!