Nice overview.

> At this size, there's no way to have a distinct upper and lowercase [...]

3x4 is feasible. I've designed the MiniGent font [1], including greek letters, numerals, punctuation, math, currency, and emojis. Written a whole LaTeX typesetter [2] around it. While not perfect, I'm amazed by how far you can take it arranging just a few pixels.

[1] https://gurki.github.io/pixeltex/minigent/ [2] https://gurki.github.io/pixeltex/pixeltex/