I don't know why it bothers me so much to see people saying, "I have a medical reason that means I MUST have early dining."
No you don't.
You have a medical issue that impacts you if you eat too soon to going to bed. No one has a medical condition that requires them to eat at 5:30 pm shipboard time.
While there are some people who really enjoy late dining, most people prefer early dining. If early dining is a show-stopper for you, reserve your cruise when the booking window opens and early dining is wide open. If you have to wait until closer to the cruise to book for whatever reason (work schedule uncertainty, etc.) then you have to decide if the possibility of not eating in the Main Dining Room at your preferred time makes you unwilling to book the cruise. You don't have the right to bump someone who reserved their cruise 6 months before you out of their place in the waitlist to move to early dining because you don't WANT to disrupt your schedule with late dining.
If you are stuck with late dining, you can still do the MDRs and stay up 3 hours later than you would normally do, and sleep in 3 hours later than you normally would. Yes, if you want to normally go to bed at 11pm there aren't as many things to do on the ship until 2 am, but you can always have a movie night in your room and watch old Disney classics. I'm not suggesting that it is ideal or optimal; I'm just saying that it is possible.
We booked a Thanksgiving cruise 2 years ago 10 days before sailing. We knew we risked having to eat at late dinner or to shift to the pool meal places/room service if we didn't get moved to early dining. We accepted the risk based on wanting to make a spur of the moment decision to go on a cruise. You book after everyone else books, you have to take your chances and accept that you might have to alter your sleep schedule.