You can read Harry Potter at any time, for certain definitions of the word “reading”.
As others have mentioned, aside from your WK level, you’ll need to learn a lot of vocabulary using those same kanji that is not taught in WK, some kana only vocabulary, some grammar, and some idioms and so on. Before doing that, your reading will feel more like archaeology. Which can be fun and instructive, but seldom feels like what most people think about when they use the word “reading”.
I’d recommend reading a bunch of other stuff first, so it will be more enjoyable and flow more smoothly. I’d especially recommend Native Japanese stuff. I’ve really been enjoying the ５分後に意外な結末 series recommended by Wani Kani super-user Leebo. They are nice, short and digestible stories from around 4-15 pages each and contain a decent variety of themes and styles. I’m no expert, but they seems to be that they are right around the easiest level or “real” Japanese (meaning that they are not written for students of Japanese or for young children).
I’m starting to think the best way to learn Japanese, or at least written Japanese, is to study kanji, vocabulary, and grammar until you get to the level of being able to read at the level of the５分後に意外な結末 series and then spend most of your time actually reading and looking up grammar and vocabulary as you discover it and adding to a SRS deck. Of course, by the time you get to this point, you’ll likely be at level 40+ in Wani Kani.