I have not used GRBL but
my initial thought is if you can adust the PWM output to give you 0 to 8 volts for the range of spindle speed you want
then use a LM358AN op amp to amplify the DC voltage by 1.25
John
PS
if you make it so 80% of the op-amp output is across R2 the gain will be 1/0.8 = 1.25
for example R1 = 2000 ohms and R2 = 8000 ohms
gain = 1 + 2000/8000 = 1 + 0.25 = 1.25