Have you considered not hammering the buttons when playing? If you find grinding trash mobs for extended periods of time to be a strain on your hands, that means you're supposed to stop or slow down. Yes, even by closing the game and resting. I don't think they'll ever add anything with the intent of keeping people playing longer in a single session.

I have arthritis and when my hands start to hurt I stop playing, because that's what my body is telling me to do. Listen to yours.