Remember this is a road bike that is being moved over to gravel. I wasn't going to swap the shifters but they pissed me off enough that I decided to swap them along with the bars. The RD can wait until it gets fed into the back wheel. The cranks/PM can wait until they die.
A rear derailleur?
The GRX has a better clutch too compared with the Ultegra RX I think, that could help on your 324 links chain