I've ridden a lot of bikes with trigger shifters, and I have to say that both xt and deore level trigger shifter sets have very light actions, and of course, cable and housing condition, housing routing, as well as shifter grunge factor (gummed up dirty derailleurs pivot points), all play a part in how much force is needed to shift. I guess all I can say is to go to a bike store and try some new bikes with diff levels of shifters and see how this compares to your mtb with triggers.
if the lightest action shifters are similar to your present mtb and cause a problem for your condition, then I guess grip shifters are the way to go, although your wrist issue could be a problem---all answers that only you really know.
good luck finding a solution that works for you.
djb,
Maybe a better upper level trigger shifter might work better than my other mtb shifter. I could try a few other bikes to find that out.
But below someone wasn't sure about a MTB front shifter indexing cleanly with my Sora front derailleur. Do you know anything about that??
Thanks, Tony