FFmpeg script to clean up and compress iPhone video recordings from university lectures
This is what I have come up with so far. Feel free to improve this answer liberally, with the objective of cleaning up and encoding lecture notes, within a streamlined process.

The command is run in the working directory, e.g. cd /Users/me/Downloads, and will encode all .MOV files present.

for i in *.MOV; do ffmpeg -i "$i" -c:v libx265 -preset veryslow -crf 23 -af "volume=25dB, highpass=f=200, equalizer=f=50:width_type=h:width=100:g=-15" -c:a aac -strict experimental -b:a 192k "${i%.MOV}-ENCODED.MOV"; done


The individual settings:


Video codec: libx265 (HEVC). This codec is unorthodox because it's still in development, but it's many times better than its predecessor in keeping file sizes low. VLC supports this codec for viewing the video.
Encoding speed vs. compression density: -preset veryslow. Converting videos with this preset takes very long time, but may reduce file size with around 20 % compared to the fastest setting. If you want quicker encoding, use ultrafast, fast, medium or slow.
Video quality: -crf 23. Quite good quality. Decreasing this number will increase quality but also logarithmically increase file size. This setting strikes an okay balance between file size and quality. 18 is visually lossless but takes up much space.
Volume gain: volume=25dB
High pass filter >200 Hz: highpass=f=200
EQ notch filter @ 50Hz±100Hz (this should be mitigated by the high pass filter already, but for some reason this seems to remove background noise: equalizer=f=50:width_type=h:width=100:g=-15
Audio codec: aac
Audio quality: -b:a 192k


Optional:


Lighten video curves: -vf "curves=preset=lighter"


Ideas for improvement:


A good way to gate audio to filter out background noise and keep only the talky bits.