Really three choices as I see it. First is to wear a pack that allows adjustable torso sizing and / or has a tall enough frame where the shoulder straps are attached to the frame slightly higher than level with your shoulders (i.e. very little 'wrap') This way, after long hours and the belt slips down, there is not excessive weight on the shoulders. My Mchale LBP34 is like this and I get no shoulder fatigue whatsoever.
Second is to wear a pack with decent load lifters. This will raise the weight up off your shoulders after the belt slips down.
Third is to use a pack that has a more pronounces lunbar pad to create 'more hips' synthetically. The belt will fit better then - more snugly and will resist sliding down.
A frameless pack will feel worse for you at weights over 15lbs or so. At weights over that, a light weight internal frame or stays with a nice tall belt will feel better than any frameless pack.