While I have not used the Slimblade, I did consider it when I last shopped for trackballs and settled for the Orbit instead, which I've owned before and can use for hours at a stretch without issue. I believe the angles of the palm rest are substantially different between the two models and there have been complaints of worse ergonomic properties with the Slimblade, which was enough to dissuade me. It's something that could be adjusted with foam and cardboard(and given a nicer texture with vinyl tape) if you're willing to experiment.

I would also review desk/chair height and other major elements of the setup too. Pain in the extremities can have causation near the hips, back or shoulders so increasingly I favor looking for holistic solutions. Lately I have been doing animal crawl exercises(bear, lizard, crab) and have found that they clear up a lot of minor aches-and-pains issues, so that might be another thing to try.