I use mogrify under linux to reprocess jpg’s extracted from (BlackVue) mp4’s. I have found that the ffmpeg extraction at 90% (or so) quality produces more artefacts than doing it at 99%, then a mogrify step to 85. I suggest that 75% is a little low.
At one stage too I was finding windscreen tinting caused a colour shift on my old capture system. I rectified that with -channel RGB before -normalize. I did however fall into an trap with that as I had an annotation strip along the base of each picture, that got included in the histogram calculation!
Oh and in the linux version (don’t know about Windows), mogrify only runs on one CPU of an SMP machine (eg i5/i7). I just shelled 8 concurrent processes with different wildcards and it completes much faster.