I’d recommend trying the Japanese Graded Readers. They can be bought on White Rabbit. But they have an app as well.
If you use a program like WKstats, it will use your WK API to determine what percentage of kanji and vocab you have for each JLPT level.
These Graded Readers are levelled by JLPT level, so it’s pretty easy to determine what one should be able to read.
As for Harry Potter, I haven’t read it in Japanese personally, but I’ve heard people say N3 level at least.
According to WKstats, at level 51, a user will have learnt 100% of the Kanji for N3.
I should also add, in addition to WK, if you want to read, you should probably work on Grammar too, if you’re not already. I haven’t used it yet, but a similar program to WK but for grammar is called ‘Bunpro’. Alternatively, The Japanese Time released a series of really useful grammar dictionaries that can be used while you are reading.
Also, there are a few bookclubs on the WK forums, there’s an Absolute Beginner one as well and we’re starting a new book next week. If you’re up to that level.
Good luck!