In the long ago, when I played squash regularly (and did some coaching with beginners), I found that the size of the racquet grip, once bedded in, was crucial. If it was too small, tendonitis rapidly kicked in. If it was too large, the same thing happened!
Holding a mouse is clearly very different - raw strength is obviously not what's needed. It may be worth having a few in different sizes and changing them regularly. This will add one more variable to the repeated action and may help stave off the problem.