How to drastically reduce size of very simple video?
(This may not be considered a complete answer, no matter the Rigid policies. This may be better as a comment.)

You can try using:


H.265 video codec (very efficient — 100-second video of 1080p @60fps was just 6-7 MB even with audio, when recorded with Bandicam screen recorder, which is free).
Mono audio channel
Use a lower bit depth and bit rate for audio as well as color/video.
Take a look at how Khan Academy compresses their vids ohh, sorry, they drop the resolution
Explore the free software HandBrake. It offers variable frame rate compressions.