I have a gunfighters inc kenai for my 4" 500 S&W.
I also have the stealthgear drop leg with dual straps for it. Also has the QLS quick detach clip system
Big, heavy, gun.
The drop leg has a great backing plate and padding to keep things comfortable, but you do need to cinch those straps down. It also has a belt attachment strap, I'll say this, with the weight of my gun, that one belt attachment strap is not enough, even with dedicated gun belts. It needs another strap or a wider top, or something to distribute the weight better and not twist on the belt. With those QLS quick detach clips on it, the gun is also several inches off the body, this doesn't help the gun moving issue, it gives that big hunk of steel more leverage...
So, I bought the Safariland big paddle and their rubber strap drop thing to try on it, haven't gotten it yet though. But this means the weight and the twisting of the top strap are transferred to the paddle, which is rigid, it won't give enough for there to be an issue... I don't think.
I have an inkling that will be a great solution.
Next, the drop leg configuration is quick, I have long arms, it is great to draw from this thing.
Another, weird but maybe just weird enough to work option.
Look at fast draw leather rigs, like old west style fast draw rigs with a big belt. Might seem weird but many of them are worn lower, so your pack hip belt is fine, and further, they drop the holster down so you also have a lower draw there, so it is out of the way of your pack and easier to draw. Further, they are fast, obviously, but still notable. Big belt helps ensure everything is secure, depending upon how low the holster drop is they may have a little leather string you loop around your leg.
Thing is, that string is just to make sure the holster doesn't tilt on draw, it isn't there to hold up your holster, so you don't have the "tourniquet" effect like many complain of with drop legs.
Sounds like a weird solution, but they are neat, and it may be something that interests you, they can be spendy, but you can get what you want, and handmade quality. It's a quick draw hip rig where everything sits low enough you've cleared your pack.