These are free graded readers:
They are designed for learners of japanese so you can use them to assess your skill. Start with level 0 books when you reach wanikani lvl 5, and know a little grammar. A little would be at least は、を and に particles, masu and past forms, and sentence structure.
If it’s too easy, then go for the next level! If it’s too hard, then try again when you know a little more!
Then get overconfident and try any book for elementary school kids and feel like you know nothing 