@seacruise9, On a sold-out cruise to Norway in August 2023, my partner and I generally walked in around 7-7.30pm daily and were seated at a table for two within five minutes. At the start of the cruise, we made MDR reservations by calling Reservations the day of but noticed that of the two queues at the MDR entrance, the queue for reservations was longer than the queue for walk-ins.
I agree that the option to make MDR reservations was not available via Navigator prior to and during the cruise itself - the only ways were to speak to Reservations onboard, either in person or via phone. In general, I don't think you should have an issue with waiting too long for tables; have a great cruise!