I've experienced some such issues, too. And indeed, I think it was mouse related, but maybe not so much the mouse itself as the fact that it was too far out.

I've switched to more compact keyboards (a MS sculpt ergonomic) and it went away.

That sounds like a likely pain point here to me too - I've settled on using a Logitech MX Ergo trackball sitting between the two halves of my split keyboard. No more stretching to move the mouse!

I've had good luck with switching the mouse between the left and right hands. It takes a few weeks to get used to, but helps a lot with spreading the strain a bit more evenly between the two hands.