Strength is a general adaptation not a specific one. With that in mind, using a backpack to actually get stronger is a terribly inefficient way to get there. Don’t get me wrong, hiking uphill with a pack is necessary to be in shape for hiking uphill with a pack, but you are training the sport specific, muscular endurance (the ability to perform a submaximal movement over and over again) aspect when you do that.
Since hiking with a pack, even a super heavy pack of 100#+, is a submaximal affair (it can’t be maximal by definition because that can’t be sustained for hours on end) and strength is a general adaptation, strength training with a barbell is the most efficient way to get the body stronger and you shouldn’t be thinking in terms of muscle isolation rather fundamental movement patterns: squat, press, pull. Put your time under the barbell with heavy weight and low reps to build strength, put your time with the pack on rucking to build muscular endurance. If you desire strong hips, you must squat heavy. No way around it as everything else: lunges, hip hinges, leg presses, squatting with a pack on, bands etc are submaximal variations of the parent movement, the squat. you’re thinking, “I need stronger hips”, but what you should be thinking is, “I need a stronger body” because increased strength is more effectively achieved by stressing the entire system vs isolating muscle groups. The specific part of this equation is then using those strong muscles to do a submaximal task over and over again for hours on end. That specific adaptation, rucking, is achieved by actually rucking and during the process of developing this sport specific muscular endurance, you develop the necessary capillary density to perform the task over and over again.