I would think all cruise lines have released their summer 2024 schedules by now.
There is no magic time to book. We don't know where you live, as various countries have differing rules on costs and abilities to change, cancel or re-fare bookings. In the US, typically, you can re-fare if the price goes down prior to final payment. After final payment, you can seek an OBC for some of the difference if prices go down.
As to cruise line, we don't know you, your tastes or interests, so we can't answer if you should stay with NCL. Yes, waiting to see if you like NCL, and perhaps booking on board for a perk or discount is an option. You can do a lot of research on the NCL board to find out what benefits there might be to booking on board.