I don't think it works that way. Plants generally also need oxygen, if only at night or relatively little. So we're basically up to Oxygen and CO2, and Nitrogen

I don't think it works that way. Plants generally also need oxygen, if only at night or relatively little. So we're basically up to Oxygen and CO2, and Nitrogen might not be entirely avoidable either depending on how the plants are supposed to get their Nitrogen. Water vapor will also be necessary or unavoidable... The pressure needs to be high enough to prevent boiling and less dramatic effects on water or similar matter. There's a reason nothing grows above a certain altitude. That's why I asked about a usable atmosphere... And anyone living on Mars would probably have acclimated to a certain atmospheric pressure. Maybe it is beneficial for them to be "altitude adapted" or maybe it's not (oxygen capacity vs risk of embolisms).