I've made tents and packs. This pack is way easier than a tent. I've tried to keep it easy to construct and the process easily breaks down into simple, independent steps. The main parts are made separately and then everything is lashed together at the end. For experimenting it is very easy to swap waist belt, bag, vertical spars, shoulder straps, etc. So you can make one part at a time and use any existing waist belts, bags, shoulder pads, etc. you might have until you can get around to making new ones. In other words you can approach the project incrementally.
I haven't made a sleeping bag but I'm guessing it is more difficult than either a tent or a pack.
So after you finish the sleeping bag it will be all down hill for you difficulty wise.