> Just breathing air with higher CO2 concentration already isn’t that great.

Or less O2. I wonder if there any study that does show any impact already or these are still speculations?

There isn't less O2. Even if all plants on earth disappeared (and animals somehow survived that), it would take millions of years before there's any measurable impact on O2 concentrations.

Moreover, there's no physiological impact whatsoever until you drop several percent.

O2 does decrease and is measurable: https://www.oxygenlevels.org/

I think this is a more marketable concern than CO2 as without O2 most know that you can’t live.

Those are meaningless changes. It's less than the effect of walking up a small hill in terms of inspired oxygen partial pressure.