Well as I said a DC motor has better low speed torque. Or a modern induction motor with a constant torque VFD which adjusts the drive voltage and current as well as frequency to give good torque at low speeds. These exist but I don't know much about them except they exist. You might also have to provide a cooling fan.

TBH I'm not sure why you want to use low speed except for tapping? With manual mills the argument for having a low speed is to use big cutters and carve out lots of metal so you don't spend hours twiddling handwheels, but with CNC you can use smaller cutters at higher speeds (with cooling if needed) as the machine does all the boring stuff. Actually since I got the Novamill I started to use higher speeds on the big mill too and deeper cuts and the cutters are happier for it.