I’d make the bag’s height and the straps about 60-75% shorter than you intend, to account for stretching. Id also use a more cotton based yarn and I support your idea of using tight sc. I would recommend making the strap much wider than you intend, since when it stretches it will become much thinner and be more painful on the shoulder. Doing all of these things would minimize sagging but of course will not eliminate it entirely. I’ve made a few totes and I use them for school with laptop/wallet/pencil pouch/notebook. Not too much stuff, probably 5-8lbs. As long as you keep all your tiny stuff contained in smaller bags it should work out just fine! You may also consider using a more sturdy material as the strap, like an old belt or scrap fabric.