Why not just use a GRX RD-RX812, which will work with a 11-42 cassette?
SHIMANO GRX Rear Derailleur 11-speed (42T max low sprocket) | SHIMANO BIKE-EU
The GRX RD is $120. Wolf Tooth Tanpan is $45. XT RD-M8000 is $65. $45 + $65 = $110, so cost-wise it is a wash.
Last edited by SoSmellyAir; 02-23-23 at 10:53 AM.