[nvln]

I was having similar issues a couple of years back. I'm now using 2 trackballs - Expert Mouse and Vertical trackball to distribute the movement. The pain hasn't come back and I enjoy having dual mice with some extra buttons.

I tried using the expert mouse between my ergodox splits, but the movement was even more awkward as I had to bring my elbow closer to my abdomen. YMMV.

Over the years, adding variety/variability has helped me more than many specific solutions. Seems obvious in hindsight given the R in RSI :D