fascinating that when you tell people "you have to learn the rules to break them" when talking about drawing/painting etc everyone nods and agrees but the second you say "you have to read books if you want to write better" there's a horde of contrarians begging to be the wrongest people ever all of a sudden
if one more person in the notes of this post says "omg op has clearly never talked to a beginner artist!!" as if im not a literal professional artist and was making a broad point........ this time im really gonna do it
and reading only fanfiction does not count as reading books to get better at writing. Fanfiction, where everyone reading it already knows who the characters are and what their relationships are and what the setting is and all that crap, is not the same thing as reading an original self-contained story.
If you only read fanfiction, you will not be able to write original stories well.
Reading books does not have to be expensive. It can be completely free. Project Gutenberg exists. So does the Internet Archive's lending library.
Fanfiction works in ways that original fiction doesn't. You can't write original fiction like it's fanfiction if you want your story to actually be well written and interesting.
I think it's important to point out why fanfiction is different - so much context and interest hooks depend on you already being interested in the source. You are familiar with these characters and therefore don't need to have the basics built into the story. With original work, you need all that underlying character development to be in the story you're writing to make us want to read it. You could 'cheat' and give us a modern Sherlock Holmes - but we still need your version explored in narrative to actually care about it. Dr. House is more than just Sherlock Holmes in a hospital, he has his own issues and foils and quirks. The Holmes from the TV show Sherlock and Elementary are vastly different takes on the same character. You need to read original fiction in order to learn how to do that in your own original fiction. Fanfic is great, but it intentional has gaps that make it a different form.