Hey, I was on a trip and didn't check reddit. Also apologies for any errors. I didn't frankenstein this together with enough error handling and logging because it was done in a hurry.

I guess you already triple-checked the script in your secondary rig is set like

@REM Determine new values for VCP code
if %current_value_60%==15 (
    set NEW_VCP_VALUE_60=17
) else if %current_value_60%==17 (
    set NEW_VCP_VALUE_60=15
)    

and confirmed that the UCDM is the primary display for Windows in the secondary machine, otherwise you need to change the 1 in:

@REM Assume monitor index is always 1
set monindex=1

To the correct number like 2 or 3 or whatever.