Any experience with Gradus M1 PRO CNC or other GRBL based stepper drivers?

Long story short, I have a modified ShapeOko2 with a Dewalt trim router. It looks like I have access to some extra NEMA 23 stepper motors free from work-- and who could turn that down?

My current stepper controls is a Arduino Uno strapped to a Stepper Shield (GRBLShield). I'd like to upgrade the stepper control to something with 4 separate stepper drivers (instead of wiring the two steppers for one of my axis together to one driver), and the more amps it can supply the better. My current board only has 3 drivers and I think tops out at 2-2.5 amps per stepper driver.

Does anyone have any suggestions of what would work? I saw the Gradus M1 Pro CNC-- 4 spots for drivers and claims that 4A per driver is possible (though I don't see any available right now or I may have already pulled the trigger), but really I've never used anything else and don't know what else is out there. I'd like to continue with a GRBL based controller, but that's more for because I am familiar with the platform than anything else-- and it has performed great for me so far. I do not currently have a serial port available, so a USB connection to my shop computer is a must.

Thank you for your input!

I will note that there are 3A and 48V drivers available --- have you priced out what a power supply for that will cost? On an SO2, I don't think one would see a real benefit from driving the motors at 4A vs. 3A --- the one nice thing would be having more headroom for tuning the current, but I suspect you're going to want them a bit lower anyway.