Manual mode, 1080p, ISO 100, adjust shutter speed so that I’m close to zero on exposure meter. Typically using an ND16 filter. Post process in iMovie and export in 1080p.
Ok but your not specifying color balance setting or shutter speed ... is it around 1/240th ? (2 times frame rate)
I'm guessing your closer to 1/80 th.
You don't want to adjust shutter speed to match a ND filter.
You want to adjust the ND filter to achieve a target shutter speed.
Target for 120FPS should be about 1/240th
target for 24FPS should be about 1/48th
By setting it this way for video it allow very slight motion blur between frames for normal slow moving subjects or slow camera movements.
For fast motion increase the shutter speed slightly eg . 24FPS set to 1/60th or 1/80th
120FPS set to something like 1/320
This will sharpen the image a bit.
Experiment to suit your subject and/or camera motion speed.
Also for post you didn't specify codec, frame rate, bit rate or any filtering, color balancing etc.
These may also affect the perceived sharpness.