I had very similar symptoms as you a few months ago. Check your hips! Good hips= stable hips= stable back= stable shoulder. I’ve had shoulder stability issues and neck pain purely because I couldn’t properly stabilize my hips and trunk. This is extremely important especially when we sit. No matter how much mobility you do in your shoulder and neck it will always round is stability isn’t acquired in the hips and trunk. If your center of gravity is off then everything else will compensate.