@tmhna A lab or a golden sounds like a very good match. They fit your exercise plans, they're trainable, friendly, eager to please. Young ones can be a bit crazy though.
Beware that there is a split between working and show lines in lans and goldens. A show line is a much better fit for you. Working lines are quite intense.
Labs also are not supposed to be fat. They really like food and people overfeed and under exercise them. If you feed your dog the appropriate amount of food for their size and activity level, they will not be fat (unless they have a health issue like hypothyroid, but then the health issue needs to be treated).
Staffies aren't quite what you're looking for. They're impulsive, have a high prey drive, can be a bit stubborn, and are dog selective/aggressive.
how true is it that staffys are agressive with other dogs?
They are prone to same sex aggression, dog selectivity, and general dog aggression.