AO3 is a really good one. It has an option that always you to mark your as Mature (as a write) which makes it so that it’ll ask if you’re sure you want to enter the story (for readers). It also has a really good filtering system so it makes it easier to find stories in set parameters because you can filter what tags you use to look (and you can use more than one tag if you go to the filters button after you search your initial tag). Honestly, one of my favorite sites to read on.
There is also Fanfiction.net which isn’t bad. I’m not on it very often but I do go occasionally. Like ao3, it has a filtering system that allows you to find results more within your parameters but it also adds the unique filter of letting you search words in the author’s name or the story’s title or summery.
The last one I can recommend is Quotev, although I haven’t been on it in a while, it does offer a variety of things from quizzes to stories to private conversations. It’s basically a social platform akin to Instagram or X but with more story writing involved.