The ideal setup is probably CO2 levels quite a bit higher than today’s, but with a sunshade filtering out UV-B wavelengths. All of earth would be habitable, but without major weather swings.

Ideal for plants is bad for us.

Um.. no? I don't know what you might mean here.

Higher CO2 levels will result in vastly more arable land, mostly from the reduction of deserts and the warming of permafrost. Higher CO2 levels make agricultural crops grow faster and bigger. Higher CO2 levels (within the range I was talking about) has no effect on human respiration. Ideal for plants is good for us too.

"For most crops the saturation point will be reached at about 1,000–1,300 ppm under ideal circumstances"

- http://www.omafra.gov.on.ca/english/crops/facts/00-077.htm

"They found that if the outdoor CO2 concentrations do rise to 930 ppm, that would nudge the indoor concentrations to a harmful 1,400 ppm.

…

In fact, at 1,400 ppm, CO2 concentrations may decrease basic decision-making ability by 25%, and complex strategic thinking by around 50%, the authors found."

- https://penntoday.upenn.edu/news/continued-CO2-emissions-wil...

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 ;)