I went ahead and bought the Shimano GRX RD-RX400 rear derailleur. I have heard that the RD-RX810, which is rated for 34 max works well with an 11-40 cassette. I am hoping that the GRX RD-RX400 (which is rated for a 36 max) will clear the 11-42. Will report back once it arrives and I install.