How do you have your X&Y accels and junction deviation set? Here is how we have our grbl machines set.
$11=0.005 (junction deviation, mm)
$120=12000.000 (x accel, mm/sec^2)
$121=12000.000 (y accel, mm/sec^2)
A higher FRC percentage will make the over all shades burn darker, so try using 20% instead. The smaller the focal point is and less the Pixel Resolution (Step over) used, it will engrave with more sharpness & contrast.