In addition to the advice already given:
Each dinner menu also has a "lighter fare" option (usually on the lower left side of the menu). The food is ligher and prepared more simply. There is a chicken meal, a steak meal, and some other options. We found ourselves often resorting to this menu because the main entrees were not things we wanted (either because of allergies or food we don't like).
You can mix and match parts. So, if you want the veggie paired with Entre A to go with your Entre B, you can ask for that
Also, you are under no obligation to eat each course. We would often have soup but skip the salad and appetizer, then have a lighter fare entree and dessert. And bread. We LOVED the bread (because of allergies they made me safe sourdough bread rolls; I loved it and once our waiter clued in to that he made sure we had at least one LARGE basket of the rolls every night and was able to get more if we made it through the first
). More than that was way too much food for us.
Finally, you may be able to order the next night's dinner at dinner. This gives the chefs time to prepare special foods (and, for food allergies, safe foods) and gives them time to adapt things when needed. So, for example, you'd be able to order say a very plain chicken breast and steamed veggies. And if you wanted the soup from tonight tomorrow, they can often do that too.
I really suggest contacting
DCL's special services team. They are the shoreside team that deals with special diet and other arrangements. You could have a good conversation with them about your diet requirements and how to best have them accommodated. I have found them great to work with (I have multiple food allergies).
If you contact special services, they may suggest that on the day you board the ship you go to the dining session for people who want to change their dining reservations. The time and location is noted in the Navigator but it is usually between about 1300-1500 -- you want to go in person, not to the one where you have to call. At the session you get to meet one on one with a dining services person. They can pull up your info and verify they special diet info they have. For me with my food allergies, they then would pull out the dinner menu for that day and I would get to order dinner for myself -- this gives the chefs several hours to come up with options. It is a bit harder for the chefs to modify for that first night, so you may get e.g. plainer food, but I am pretty impressed with what they can do
Hope this helps.
SW