Yes its perfectly fine to have the same thing every morning for breakfast if thats what u choose to do. I do it myself .
Yes, it can help you stay on track every morning and even may make you feel more energized if it’s good for you. It can also save time because the specific meals becomes routine.
No, you should have variety. Even if you eat the same thing on work days, you can have something a little more involved on your days off.