Look into the Peak Design Slide. It is by far the best strap I have used. Keeping the strap across the shoulder is very secure and much more comfortable with a heavy camera than on the neck. I use it with the Peak Clutch hand strap on a D800E. I have an L Bracket mounted on the camera too, with the shoulder strap connected to the top left and the bottom of the L bracket. That puts the camera hanging comfortably at my side/back and quickly in my hand when I want it.
One of the features I like about the Peak straps is the quick release buttons let me remove the strap quickly for tripod use, or to switch it to another camera. The quick length adjustment is also nice, but I generally just leave it alone once it's set.
There used to be a camera strap, and one for a bag which had a degree of stretch and were quite wide. They were claimed to aleviate the exact problem you have. Unfortunately I cannot remember the name but there may be someone from UK who uses the forum will remember