Lvl 50 Geralt. Finished B&W main quest. Few side quests remaining. Is this a good level to start NG+?

It's fine. The monsters and equipment scale to (roughly) your level, so go right ahead.

Yeah, honestly the NG+ isn't difficult. In fact since you've already beaten the game once and your gameplay skills have improved, the NG+ run should feel much easier, at least it did for me.