The trouble with non perishable foods is so much of it is high sugar/white flour carbs. All they do is make you blood sugar spike, then you crash and get hungry again. A better breakfast option is eggs, whole fruits, high fiber carbs, and yogurt. So low refined sugar granola bars are among your better bets.
Dried fruit is among the better suitcase options. Nowadays you can find dried blueberries, dried cranberries, dried apples (I've even seen a Mickey version of freeze dired fruit in individual sized packets!). Another option - kind of like granola - would be dried nuts or sunflower seeds or the like. Some people like peanut butter.
I also like to just wait and pick stuff up when we are there. If you are looking to save time, pick up a bunch of fresh fruit in advance in your resort food court or gift shop. Most of the resort gift shops carry a few grocery items. They also sell fruit in both AK and MK ata fruit stand. That's often what we go for quick ehalthy food that also happens to be inexpensive. It doesn't take much time to grab an apple, and they aren't expensive. Sure it's slightly more than the cost of bringing it from home - but it isn't mushed/bruised either.
Also in the resort gift shops is milk. orange juice. Then again, the fridges in the rooms are not all that cold. I have tried using the ultra pasturized milk boxes. (parmalat ins one brand, Horizon makes them too) They do work, because they don't have to be refrigerated and come in cup sized portions, but maybe not so great if you are flying. (we like coffee in the am with milk)
Coffee is VERY fragrant and really has to wrapped well, or your whole suitcase will smell like coffee!
We actually find the dining plan is TOO much food. We especially can't eat all the desserts! We do split meals, and where possible I try to get yogurt as the 'dessert.' WDW doesn't have the best yogurt, but it's better than mediocre prepackaged cookies or chocolate cake. I wish they carried plain Greek yogurt myself.
If saving time is you consideration, here's what we've tried: DH gets food while I get ready! We beat the food court rush, and either eat while in the room still or carry it with us and eat it on the way to the bus stop. Even if you just split one portion of eggs and each have some sort of bread with it - that really holds your appetite better than bread alone.