Droitaubut, thanks for sharing the above link! That's exactly what I've wanted for a number of my Frankenbike projects as the road FD's don't have the range to work with a MTB triple crankset. Instead of this I've fashioned my own shims using a few layers of old inner tubes which are glued together.
Without knowing what bike model I feel like I'm just taking SWAG's...
The Shimano GRX crankset appears to only be available with traditional road BB's so that would indicate a FD shim shouldn't be needed.
By not going to 12 speed, parts are MUCH less expensive and because 10 speed chains and cranksets are compatible it's keep the cost down and makes it a lot easier to find parts if something breaks while traveling.
As for interchanging SRAM and Shimano cassettes - as long as you're not using the Shimano 11-34 road cassette, then there is no difference in cassette width. With 10 speed, the SRAM and Shimano cassette bodies differed by 1.0 mm. All the Shimano 11 speed road and mountain cassettes have the same width EXCEPT for the 11-34 which was designed to fit onto a 10 speed freehub body. This is awesome for those of us with 10 speed wheels which we'd like to use for gravel and want that lower gear.
So many "standards", so little room left in my brain...
I wasn't the one who provided the link, so credits to drwelby for that. GRX is with traditional road BB's, but q-factor of crankset is 5mm more, and as such FD is adjusted to chainline of +2.5mm as well.
So how to use the Shimano 11-34 11-speed cassette on a wheel, need a spacer?