If you’re up to it, I posted a links to some of the ones that were originally web novels.
The reading experience isn’t as nice as a book but it’s free. 
Otherwise, I would recommend BookWalker.jp
I would actually not recommend the LN for your level but go for it if you want a challenge. I started it pretty early in my journey but I liked the anime so much that I slogged through it.
The main thing is that the narrator is a middle aged otaku and the novel descriptions especially are filled with colloquialisms, katakana words, and the contents of a full adult vocabulary.
Here’s a sample of the vocab lookups I did just for the first volume:
But, like I said, I loved the world and the story so much, so by all means give it a shot. 