Book 2 was the highlight.
I don't necessarily disagree. It was certainly more focused, which I feel as if Book 3 could've benefited from. It felt like a nice, tight little package (speaking of which, is Jinora on the table in book 4? ...nevermind). Book 3 was really good purely in terms of writing, though. I didn't particularly care about the whole snake woman plot until Book 3 began progressing it, and Katara returned and added a whole load of mystery to the story. It did a good job of turning Four Elements Trainer into a proper and somewhat compelling narrative.
Also, to be fair, every Book but 2 had a sort of 'Grind it out' mentality. Like, keep doing the same activity over and over to raise some arbitrary bar. Content is locked behind simple clicks. Book 2, the content was locked behind battles that you had to use at least half your brain for. I mean, it wasn't Total War, but still pretty decent. I think Mity tried to do the same for Book 3 in the dungeons, but eh.