. . . a lot of backpacks put pressure on my upper back and shoulders in a migraine-inducing way. . .
I come here from a wilderness backpacking background
(albeit many years ago) and am amazed by the people I meet who do not understand how a backpack actually works. It is, I suppose, understandable that some Pilgrims look at backpacks as not much more than a carry bag. Shoulder and upper back pain are caused by carrying the pack with your back, not your hips. Despite its name, a backpack is actually carried on your hips. The shoulder straps keep it steady, but the hips carry the weight.
Think of a backpack as a machine that you wear. To CARRY cargo it has a bag. To TRANSFER the cargo weight to your hips it must have some sort of internal frame ... there are various frame types, which include flexible sheets, rigid sheets, wire hoops, articulated wire double hoops
(x, figure 8, butterfly shapes), or an external frame. Each frame type has its own advantages.
Many small packs do NOT have a frame. AVOID THESE at all cost!!! These are glofied book bags that kids use to carry books short distances.
GENERALLY SPEAKING: The more rigid the frame, the greater the comfort for heavy loads. The more flexible the frame the less load that can be carried.
There are other factors, but frame is critical for load carrying.
Most pilgrims carry a very light load. Typically 20# or less. Conseqently a light duty pack
with an internal frame is suitable for most pilgrims. The next step is to find the right torso size. Go to a real outdoor store (REI, Moosejaw, etc) and get properly fitted for a backpack. Tell them to put 15# or 20# in the pack while you test it.
Getting the TORSO size is the critical step and that is why you need to go to a good store where they will fit you for a backpack. Yes, just like you get fitted for a dress, you need to get fitted for a backpack. If the frame is doing its job, and the torso length is properly sized, you should feel essentially no weight on your shoulders!!!
You also need to have them teach you how to put on the pack. There is a proper sequence of steps in which strap/belt to adjust first, and which to do last.