If I were in your position, I would look at the motors and run the numbers.
1. What amps are they rated for.
2. What is the inductance of these motors.
IF the motors are rated for much more than 3.5 amps, then the 251 is probably not satisfactory for this project unless you want to seriously derate the motor.
If you run the 32*sqrt of inductance and the number comes up seriously greater than ~50V then again, this driver is not a good choice.
There are all kinds of tradeoffs that you have to look at. You are correct that the holding power is ~to the current and the speed is ~to voltage as I understand it.
Never having milled wax, I can't say for sure, but I would think that you want a lot a acceleration and speed, but agree that it will not take a lot of power.
Hopefully someone with more knowledge than I will make more sense.