I’ve been using a trackball, left-handed, at work for the last few weeks. I’ve found that it helped with my shoulder problems a bit, and it wasn’t that hard to use. But I never got really good at precise positioning with it. So now I’m trying a vertical mouse (right-handed). I’m using this cheap one from Anker. A friend recommended an Evoluent VerticalMouse, but that’s $90, compared to less than $20 for the Anker, so I decided to give the cheap one a try first. It’s working OK so far, though I’ve noticed that double-clicking is more trouble than with a regular mouse, since clicking the button tends to move the mouse a little, and if it moves between clicks, it doesn’t register as a double-click. So I’m getting used to that.
I may try to switch back and forth between the trackball and the mouse occasionally, just to keep myself from relying on one hand or the other for too long. And I think I might try a trackpad at some point too.