ZDoom

My understanding is that the HUD fonts were changed to support UNICODE, at the expense of the traditional DOOM HUD font; however, the replacement is very very hard to read; it's all squiggly curves in red on what are often (in game) dark backgrounds.

I can find no option to change the HUD font; is that because I am dumb and can't find it, or because there isn't one? Is there any way to replace it with a clean sans-serif typeface, preferably in upper case and brighter colours?

Currently there is no way. You couldn't do anything about the colors anyway because the raw font is 1-bit data, all color is done programmatically.