Switching to a tackball did the most good for my wrists and forearms. I also bought the curved kinesis keyboard. It takes a while to,get used to the kinesis key layout, but now that I have I type insanely fast on it.
Just to be definitive, I suggested the kinesis Advantage2. It should help with your non-mouse hand. There are plenty of reviews of people who claim it saved their careers from RSI.
Good luck! I’ve not had any rsi pain since switching to the advantage plus a trackball. I hope it works similarly for you!