I use a 10:1 precision planetary on a NEMA17 motor.  Adding a planetary will slow down the slew rate, but then everything is a tradeoff anyway.  I am not trying to swing at a baseball with my scope, so I live with the speed I have.

A couple of ideas in order of ease:

• Go to a higher voltage.  Go out and buy a dedicated 24-30V power supply.  The Onstep can handle 24V without a problem.  Don't bother with 18V, it probably won't be enough.  This alone may solve your problem.
• The steppers come in different lengths.  The longer the motor, the more windings, the more torque.  Be sure you have the longest motor in the current NEMA frame you can get.  I think Oriental Motor has one of the highest torques available in a 2" length.
• Add a gearbox.  This is a torque multiplier. A 5:1 planetary will multiply your torque approximately 5X, but it will slow your maximum speed by 5X also.
• Go to a larger NEMA frame.  Self explanatory

https://www.omc-stepperonline.com/ple-series-planetary-gearbox-gear-ratio-5-1-backlash-15-arc-min-for-nema-17-stepper-motor-ple17-g5

Bob