I am not sure that Diamonds and Plats get priority once you have checked in for YTD, Diamonds and Plats get priority as far as your initial dining choice (Early, Late, or YTD), but I am almost certain that getting seated for YTD is based on when you check in and the size of your party, not your status.
Journeys cruises have longer waits because they skew older, and older people (myself included 😁) tend to want to eat earlier. As a result, there is a huge flurry of check-ins right when the system opens up, resulting in longer waits if you want to eat before 7:00 or so.