Well... it turns out that STANCE MATTERS. On my Ninebot One S2, I changed my foot position so that my heels were on the back of the plate (or slightly over sometimes by 8mm) so my toes were completly over the front plate (yes, it feels wierd initially)... And guess what, my control of the wheel increased (i felt comfortable going 12MPH instead of 9MPH) and my foot fatigue decreased by about 40%.

Just to mention that i wear size 10 UK trainers :-)