I've got slight back issues, so something that won't destroy my back in the long run would be nice.

The best ways to avoid straining your back are either to carry as little weight as possible in your pack (2 kg or less), or else find a backpack that has a belt that effectively transfers the weight of your pack so that it is fully supported by your pelvic girdle (your hip bones) and the shoulder straps are mainly preventing the pack from falling backwards.

As for sorting among the many hundreds of packs out there that are capable of carrying the load you describe, the most important factor is something we cannot help you much with: the pack must fit you comfortably when it is fully loaded with the items you typically plan to carry. Some inexpensive packs are well made and durable, and would be perfectly suited to your needs, but no one here will be conversant with more than a few packs and none of us can tell you how any one of them will fit you.

The best suggestion I can make is load them up, try them on, and examine the materials and workmanship in detail. As soon as you find one that works at a price you are willing to pay, your task is done. Good luck!