Ok I didn't mean to imply maximally ideal. 300-400ppm CO2 would be a tremendous boon to both agriculture and most biodiverse wild ecosystems (e.g. jungles & forest tundra, not deserts). It would also warm polar regions more than it does equatorial regions, making northern Alaska, Canada, Russia, and Scandinavia more tolerable to large-scale, year-round human habitation, as well as Antarctica and Greenland (the resulting sea level rise being an issue tho).

300 ppm is the normal interglacial high, we’ve been above 400 ppm since about 2015: https://climate.nasa.gov/vital-signs/carbon-dioxide/

So yes, going down to that would be an improvement ;)