When buying the pre cruise dining package that requires booking once you get onboard make that your First Stop (before the buffet) at Any of the specialty restaurants Or the main dining room where you can book All your preferences at one stop.
like a dummy I once thought you had to go to each venue to book dining times but lucky for me at my first stop the person there ask if I had any Other reservation requests for other restaurants!
I see First night discounts on most cruises but only for Late (8:30pm) reservations and we are early diners.