I had pretty terrible RSI (and even more terrible handwriting) and could write much without cramping up or pain. What helped for me was teaching myself proper cursive and fountain pens. Rather than clutching a ballpoint and marking with jerky finger/wrist movements, I now use my arm for larger movements and let the pen glide. It’s helped tremendously. It’s slow going at first but keep at it. Plus fountain pens are pretty fucking cool. Also, paper matters too; but paper and notebooks are another fun rabbit hole.