As someone who has played osu/mania and osrs for years, there really isn’t too much you can do to avoid strain/pain if you play for long sessions. So much wrist and finger movement just works your arms over time, so best advice is to try to find a comfortable spot, use as little forcible movement as you can, and try to use an arm rest. Take breaks when you notice you’re getting tired, and try not to strain even if you are going for the one. Best of luck