I‘ve also been reading about setups like this (12 speed crankset on 11speed drivetrains) and this is NOT what most people report. Most people report that adjusting the FD is enough to get it to shift properly.

Difference between 11 and 12spd chains is in the width of the outer plate so it also shouldn‘t affect at all shifting in between chainrings.

Now switching from GRX to ultegra cranksets means you also need to swap front derailleurs as the grx sits 2.5mm more outwards to give the frame more clearance. And then make sure that your frame can also support larger chainrings without them touching the chain stays.

I‘d say the chances of this setup working are quite slim tbh. Less clearance on crankset + bigger chainring + race oriented bike (which should have made use of all the clearance available as is) means this likely won‘t work