GRBL doesn’t have this level of control - it’s either constant power for the whole cut, or ramped based on speed. DSP controllers, like Ruida, allow setting both a low and high (min/max) power for the ramp, but GRBL has no method for this.
Increasing your acceleration can help with this, and on a normal Line layer there’s an ‘overcut’ option that will got just a little farther around closed shapes, and that would help here too.
The $31 parameter works backwards to what you think it does - it means “what value from the software means zero power on the controller” - it’s used to map RPM ranges for rotary spindles, so you can map the range 3000 to 15000, for example, if you know your spindle won’t run slower than 3000 RPM.
Newer versions of GRBL (1.1h, maybe?) have some additional parameters for the minimum PWM output value, and Smoothieware has a config setting for this too.