Because you mention it is for the express purpose of shock absorption: Shock Stop Stem

You should not be feeling hand pain under any circumstancesBein's that GRX ergonomics are so well crafted, I should think that it's not the GRX shifters at all. You might try swapping a few spacers around on the stem...maybe lower a bit, but not so low a back or neck problem arises &/or combine with a little bit of bar tilt one way or the othet to place the force on your hands a little bit differently. It's free to do & there is no downside, to try. Move it back if it doesn't work.

Fitters don't know everything; They just get you really close. It can be an iterative process until everything is dialed between you & your ever-changing/improving fitness & the bike. Now that you "have your numbers," it's ok to experiment around a bit to fine tune any issues that may become exposed now that the big stuff is out of the way.

Good luck.