There are two things you can do to your sling, to reduce finger fatigue.
You can try using a wrist loop instead of a finger loop. This is less likely to be effective, in my opinion, because a finger loop doesn't need much muscle strength, it just uses the tensile strength of your finger.
You can make your release knot or toggle, bigger. A bigger release knot means more surface in contact with your skin, so you are using friction more and muscle strength less. I started out with a double-overhand knot, then I tried a monkey's-fist knot (which was TOO big). Most of my slings have a
heaving multiple-overhand knot on the end of the release cord (5 to 7 turns). Recently I have tried
TechStuf's folded-leather release tab
(third picture), and I like it!
EDIT: Sorry, wrong name for the knot. Either a heaving knot or a multiple-overhand knot works as a release knot, and the heaving knot is actually easier to tie.