It is actually not "smaller". It has more splines to distribute the torque. It also has a carrier on the cassette to eliminate gouging.
I have bought DT Swiss H1700 wheels designed for ebikes which comes with micro spline driver so I am confident that part is ok.
Shimano 12sp seems like the perfect emtb driveline. It actually works better to change under load. Also running XT or XTR shifting with a SLX Chain/Cassette is very cost effective vs SRAM cassettes