Much depends on how frequently you can restock supplies along the route. Also A 2kg down sleeping bag must be really voluminous - our old ME Iceline bags were for serious winter conditions and were around 1.8kg, one of those would almost half-fill our old quite big packs!. The Atmos packs are relatively heavy by modern standards, if weight is important, as it probably is on a month long trek, there are lighter choices that are as comfortable.
I agree that it's essential to try packs with your intended load, preferably in a shop where several types are available. Everything about a pack is personal and everyone is different. Osprey packs are very often seen on the hills here largely because they are ubiquitous, almost every outdoor retailer stocks them (at least the heavier ones). Again it's personal, many walkers find them excellent but many don't, including us. We knew immediately they weren't for us when we tried them on, they just didn't feel right. That much touted air gap at the back pushes the load farther away from your centre of mass which is bad for stability and biomechanics.
Unfortunately the only retailers we know that stock very light packs are all online only.