I completely agree that she needed an editor from Book 4 on. As for Book 6, it does ramble, but makes more sense when you think of the last two books in particular as two halves of one novel, which is how JKR thinks of them, according to interviews.

Apart from everything else, I just love how JKR turned Dumbledore into the Potterverse equivalent of LFN's Operations and introduced generations of kids to the concept of abeyance through Dumbledore's death, even if she used different words for it. These ideas are old hat for some, but extremely provacative for readers under twelve.