i definitely think you need to read a mix. reading fic helps you understand what you like and don't like in fandom spaces, like interpretations of your little guys, jokes, themes, etc. but ultimately fic writers aren't (usually) paid professionals that are the most polished and perfect writers in the world. there's flaws and human error and stylistic stuff that's uncommon in novels, and that's what makes fic unique. that's why it's important to also read books you know, because these are vetted by editors and they go through publishing houses and they're fine tuned to be consumed and critiqued from a literary stand point. there's an element of trust that you can put into a published book that it will be professionally written and professionally executed, whereas there's a level of connection you can put into a fic writer that their fic will be personal and lovingly written because they love the characters in a way published work can't exactly provide

but i also think that you should read fic for fun, not because you want to study it or what not. of course you can sit down and read your favourite fics to get a firmer grasp on what kind of writing style you like, but ultimately i'm not going to tell you to read stray kids fan fiction for the purposes of becoming a better writer. just read skz fic because you want to, and if you become a better writer bc of it then that's all the better