I think 6 isn't so different from 5, the relationships are very tame, there's no mention of anything more than kissing. The way the relationships are written isn't super healthy, but it's written from a teenage POV. There are no mentions or references towards sex at all.
I'd say the most disturbing things in book six are the two times Harry is immobilized and invisible under his cloak- the first time Draco Malfoy stamps on his face (on purpose) and the second, he watches Dumbledore die.
Some of the flashback scenes involving Voldemort/Tom Riddle are a bit creepy, but still in the realm of fantasy stuff.
In all honesty, I think the graveyard scene in book 4 (kill the spare) and a lot of stuff in book 5 (Umbridge torturing students, the department of mysteries) is worse. IMO book 4 is where the tone really changed towards "dark". Cedric's death, to me, is the worst in the whole series, because a) it's the first real "on screen" character death and b) it's in cold blood and shows a very callous attitude towards human life.
Book 7 is obviously different again, but most of the worst stuff happens off scene, and I actually found the deaths less impactful because there were more of them- if that makes sense? Harry walking off to die is obviously not a nice scene though.