I’m interested to see how this turns out. 0.004" backlash ain’t that bad. Wouldn’t it be cool if Marlin or GRBL could use DRO feedback to get rid of backlash? If they could, I’d go for an all ball screw rig in a heartbeat.
I had similar results as you when I was playing with lower microstepping on my primo (drv8266 @12V 1.4Arms) after changing the z from an 8mm lead to a 2mm lead (to lift a much heavier spindle). Going from 1/32 to 1/4 didn’t seem to make any difference in the max rapid speeds I could get. I swapped in 2209’s in standalone spread-cycle 1/16 mode 1.4Arms and they are slightly better. I am upgrading to 24V to get my rapids back though.
Something like this @24V (48V even better if you can afford it) would work much better than 8255’s for your nema23:
https://www.automationdirect.com/adc/shopping/catalog/motion_control/stepper_systems/stepper_drives/em542s
The cost is not in the spirit of MPCNC I think. That is stepping in to ‘big boy’ cnc territory, where costs go up quickly in general. However it’s a possible solution if you run into problems with torque.
Kev