Just want to add a couple things from my wife's own experience with dairy/gluten free on the Fantasy.
First thing the servers will ask you that first night if this is a personal preference or if it is a severe allergy. They will accommodate you either way, but if you say it is a severe Allergy, they will go the extra mile to make sure you don't make any unsafe choices. If you say it is just a preference or a sensitivity (as I think you say it is, and as it is for my wife) they will still help you avoid it, but if you want to make a choice that has a little chance of exposure, they will allow it if you ask for it.
Second thing is, they won't make you feel bad or like you are being a burden, quite the opposite, they will make you feel SPECIAL (in a good way). They made it clear that these sorts of requests are no problem and that they are pleased to do it.
Third thing, as said above, you will order the night before for that person. At some point during the meal they will bring you the next day's menu, but if you have the Nav Ap on your phone, you can look up the next days menu earlier in the meal and write down your choices. My wife started doing this about half way through our trip. She found it more convenient, and saved us time when we wanted to get going at the end of the meal.
Last thing, if your DW is going GF and DF as mine did, it will rule out ALMOST ALL desserts that they have. My wife likes fruit, and so the first night she had whatever fruit they had on hand as a dessert. After that they had her name any fruit (or combination) that they had on the ship (they could list them for her too). Each night she would pick one or two for the next night and that night, they would bring a big portion (cut up), enough for the whole table to enjoy. So this made her feel good too.