The CSS to make the terminals look like iTerm was smooth, to the point I read them as screenshots.

Keep it plain text. Regular, old, boring output is good.

Problem there is you can’t change css so at the moment the systems color preference changes thing will look bad.

Important considerations for custom formatters.

Ideally, instead of the CLI app switching to "bright green", it would pick a "bright contrasting green". So if the terminal background was dark, it would pick bright green, and for light background it would pick a darker green. There isn't CLI app implementations for this? This is similar to how you'd implement dark mode in a web app.

Colors, have been a perpetual nightmare.

I think it's safe to set a standard colour-set so that it's immediately usable, but beyond that, a user should be customising to their requirements.

Perception differs among people; many of the colours OP listed as unreadable, were barely an issue, bright yellow being the only one I could unequivocally agree on. Perhaps display type, configuration and colour calibration is an important factor, as well as individual perception, ambient conditions, brightness levels, contrast, and perhaps even more variables have a significant effect.

I've also learned, since adding an OLED Monitor to my desk alongside the IPS ones, that it's possible to have too much contrast; brightly coloured text alongside pixels that are literally off can be just as problematic to read at times, as low-contrast.

When going beyond that, the colors really need to be configurable on the application.