This isn't going to be cost effective.
But I love Frankenbike challenges!
I would probably:
ST-RS685 11 speed shimano mechanical shifters/hydro brakes
Tanpan travel agent for the XT RD (assuming it works with your specific RD)
Shimano 11 speed 11-40 cassette (will fit 10 speed hubs, the 40 "dishes" over the spokes)
Go 1X up front for cheapest/simplest. Mountain bike frame and BB spacing will introduce some other challenges at that end if you want a double.