To go even further- I think she was too ambitious about her own writing ability.

Having a series of 7 books, each tied to a school year, where the characters age over time, with the intended audience also changing over time. Sorcerer’s Stone is a book about 11 year olds, for 11 year olds. Goblet of Fire is about 14 year olds, for 14 year olds. There’s a lot of wiggle room, but that’s the baseline. Sorcerer’s Stone is a pretty simple children’s book. Prisoner of Askaban starts dealing with the history of Voldemort 's rise to power, starts dealing with more powerful banned spells that raise ethical questions, the criminal justice system, etc.

I remember when book 5 came out being heavily disappointed in it. It was just a dark and depressing slog. Half-Blood Prince was just boring- most of the book focusing too much on just teenage drama and romance. Deathly Hallows had an entirely different tone from the rest of the series and felt like bad fan fiction. All the way up to eh epilogue, where we get a glimpse of the main characters as adults that feels like really hamfosted fan service. I think Rowling was just better at writing for/about 11-14 year olds than she was 15-17 year olds.