By all means take food to your cabin, we do it all the time. They even bus the dishes out for you.
Write whatever you want on your breakfast card. Occasionally they will omit an item they can't get but they try to get it for you. We've successfully had bacon and eggs, for example. We just add to the tip if we've asked them for something special. Remember on early port days they are going to be especially busy so have a heart and walk upstairs or order pastries and fruit those days. But for a nice relaxing sea day....enjoy