I don’t think of my comments as a “solution” to your question, just my guess about a possible outcome. The “problem” with the MPCNC is that everyone does their own version.
- they pick the size
- they pick the router and the mount
- they pick the plastic and the printing parameters for that plastic
- they select what tools are used in the design and cutting pipeline
- they may select custom features
So, with all these personal choices, it is hard to see the limits of the MPCNC or how the many customizations impact the performance of the MPCNC.
Your changes seemed to be targeted at torque. At slow speed, torque determines how deep each pass can be without losing steps. Your CAM software sets the dept of cut for each pass, so the “only” thing you are gaining with increased torque is less passes (faster cutting). And there are Nema 17 stepper motors with significantly more torque than the HS4401. In fact, I’ve seen ones on Aliexpress that have twice the torque (so you could get the equivalent torque of your two-motor system in a single stepper motor without MPCNC modifications). And I seen these steppers on the forum several times, which are both inexpensive and half again the holding torque of your HS4401 steppers.
Stepper motors lose torque with increased speed. The point of significant torque loss is far faster than most CNC routing. But if you are cutting a light material like foam, or doing laser engraving, or doing pen plotting, then torque at speed may become an issue. Here is one post where higher voltages and a faster control board made a difference.