Hi all,
I have (sort of) completed my build with GRX 810 (2x mechanical).
Happy to report the shifting is fine.
I was initially afraid b/c the shifting did not work at all at the start, but after forcefully shifting maybe 50-100 times per shifter, it is now really smooth.
NOTE: when the shifting is stuck at first, just force and go at the limit [lever limit]; the ~0.5-1mm the cable is moving at the other end will make things unstuck after a while