People buy new mice to fix aim that is actually broken by a desk that is too high. Mechanics cannot overcome a shrugging shoulder for three hours.
Elbow test
Sit normally. Your forearm should rest roughly parallel to the desk with shoulders dropped. If your shoulders hike up, lower the chair or raise the desk (or add a board under the monitor / adjust chair height carefully).
Mouse reach
The mouse should sit close enough that micro-corrections use the wrist and forearm — not a stretched shoulder. Large pads help; reaching across a keyboard every flick does not.
If your shoulder burns after one hour, your setup is training injury, not aim.
Monitor
Top of the screen near eye level. Looking down for hours wrecks neck posture and makes vertical aim feel weird.
Breaks
Every hour: stand, open hands, look far away. Ten seconds is enough to keep sessions usable.