Hi, Ed - as far as carrying around a D850 with a 500 or 180-400+TC - I’ve been fine using an extra-padded tripod foot as a handle. It’s certainly not light, but entirely manageable.
My secondary system is attaching a padded strap with 2 correctly-sized wide loops, one to the front of the lens and the second to the back of the camera, with the cam/lens balanced. I put the padded strap over my shoulder and my left arm around the camera to help support it.
Sandy