There have been other threads on this, but they don't seem to have had any traction.
No one uses the backpacks because of the penalties to noise and visibility. But in real life, all hunters will use a backpack to some degree, so it's not very realistic how it works in game.
My suggestion would be to remove the noise and visibility penalties. Instead add a movement penalty tied to the weight being carried. For example, if you're carrying over 25 you cannot sprint.
The pack mule perk wouldn't be affected, as it should raise the base stat that weight penalties start from.