Is science asking this question? And does science offer a clear answer here?
CO2 levels has been higher than current levels before, like back in the Triassic. Incidentally, very few humans would like to live in such a climate. It's also worth mentioning that the Triassic was preceeded by one of the worst extinction events that ever affected the planet. An extinction event caused by very high temperatures and increases in CO2 levels.
Sure, life will survive. Probably humans as a species too. The planet won't be the same as now though.
I try to research the available science but have trouble finding clear answers. The most accurate data seems to comes from the ice samples. But that only dates back 800.000 years.
You mention the Triassic period. Co2 levels were 2000ppm during that time. That sounds like a dramatic increase (from 0.04% to 0.2%). But then I also read we already often experience such levels at home or at work. For office workspace 1000ppm is considered acceptable and official limits are set at 5000ppm (as 8-hour TWA). I also read plants will grow ~5 times as fast. More faster reforestation?
We know there will be a change. But how accurately are we able to predict what that change will look like? Is there a strong consensus about it?
Perhaps we can agree on "the best composition for humans"? In which case I humbly submit the answer is "the one we (and every animal we care about) evolved with".