>>5949738
Trial and error. I'm not an expert and really only use HandBrake for occasional memery and shrinking file sizes at work. Average Bitrate makes it easier to hit a specific file size limit, but I did a few encoding runs to find that Constant Quality 62 (two pass) would yield similar results in 5.7MB. But as they say; unused data is wasted data.
Video encoding settings were as follows: VP9, framerate same as source, average bitrate 255kbps, two pass encoding (HandBrake uses a turbo first pass), and the encoder preset at the slowest setting to eke out as much "quality" as possible at the cost of a huge time penalty. Though with all the fine detail and motion it still ends up blocky and smearing a lot of the time. Dropping the framerate as the earlier video did doesn't appear to be a bad strategy either since it's not noticeable without a direct comparison. And even then it's difficult to see.
>>5938206 is a better example where there aren't a lot of fine details or motion. This, and the shorter length, allows a bump up to 720p without too much quality compromise.
I'll add that I'm open to suggestions for improvements because as stated I'm not in any way an expert in this field.
Reference: https://trac.ffmpeg.org/wiki/Encode/VP9