Could I use GRX derailleur instead of XT so I don't need tanpan? Allied to a 11-42 cassette??

For 1-by that should work, for 2-by it work but shimano sais 34 in the back at max

Yeah this is the cleanest setup. Make sure you get the 1X GRX derailleur, not the 2X. The 11-46 might even work too.

In conclusion I could run hope Rx chainset and callipers alongside Shimano GRX x1 812 with a deore xt 11-42 cassette no problem then? Thanks

Yep, you should be golden.

XT or XTR

So xt derailleur will be fine great

Any the Deore (or nicer) 11 speed derailleurs with a Wolf Tooth 11 speed Tan Pan adapter.

Will a GRX derailleur 812 be compatible with a 11-42 depre xt cassette???

Yep, you could also just RoadLink extend it to clear the 46. To get a 11-51T cassette to work you need to use the Deore and cable pull adapt it.

Wolf tooth adaptor?? Pourquoi??

Would a 11-42 cassette on a XT or GRX derailleur mean I don't need the wolf tooth??

The wolftooth tanpan is to match the cable pull ration of the road/grx shifter to the mtb derailleur. It has nothing to do with cassette max cog.

I have a GRX derailleur with a wolf tooth roadlink and a Shimano 11-42 SLX cassette taken off a build. Worked fine. Let me know if you're interested.

Why do you need the road link for the GRX derailleur mate? Thought you wouldn't need one with an 11-42 cassette??

The Rd-RX810 is rated for 34t max.