> [...] but their coils get saturated at higher RPMs meaning they lose efficiency at highway speeds (which actually affect the range number people car about).

This seems like a big disadvantage. Highway is exactly where EVs fare worst compared to ICE cars.

I wonder if this could be solved by introducing a gearbox?

I know the new Mercedes CLA (EV) has two gears, the second gear being optimized for highway speeds. But I don't know whether it's related to this.