6 - There is no NG+ at all. Once you get the true ending, you're given the option to save over in which it rolls you back to a previous "cycle". I haven't really touched the game since I finished it since I noticed right away, it was the cycle part of the game again and I confess that part was a bit of a drag.
Otherwise, I wound up getting close to max level grinding solely on bosses. I recall finishing the game around level 92, which included fighting just about every boss on almost every cycle. You can definitely get to level 99 without much effort though since the game caps the EXP requirement per level at 70k past level 70 or so.
PSN: Neophoton Currently playing: Persona 4 Golden, Bravely Default: Flying Fairy