One of the issues with shop bought muesli is the use of dried milk powder which drives up the sugar content. You can get apps for a smartphone which have bar code scanners and provide a nutritional breakdown of a huge range of food products. Personally having done, several times, what
@bulkbiker suggested I've come to the conclusion that cereals and muesli are to be avoided.
And a lot of what you find on the cereal shelf has to be so heavily processed to give it any shelf life that by the time they've finished it has zero nutritional value. So it is then "fortified with added vitamins". If they didn't do that you might as well be eating the box it came in.
I've wondered about porridge and the difference between whole oats and rolled (sliced and flattened) oats. but never experimented since I've given up eating breakfast.