Sure, on modern Earth, ~95% of CO2 comes from the organic carbon cycle and ~3% from volcanoes.

But then 99.99% of the CO2 is CONSUMED by the carbon cycle. 95% is recycled, as above, but ~5% is lost from the biosphere by burial. Only plate tectonics permits this to return again via volcanoes etc.

So, barring human intervention, the level of CO2 in the atmosphere is determined by the balance of volcanic inputs and organic burial. As these change through climate, plate movement, and changes in biota, the atmospheric CO2 balance shifts.

On a planet WITHOUT organic burial, one would naturally expect the CO2 content of the atmosphere to be higher - say 200 times.

(You are aware that the pressure of CO2 in mars atmosphere is determined effectively by how warm or cold the polecaps get - especially the southern polecap. There is an annual cycle of over 25% of the entire atmosphere passing from vapour to solid and back to vapour again - see Tillman's Viking work. If Mars were warmer, it could have even more CO2 in its atmosphere)

Where does Ozone on mars come from? From photolytic CO2 and H2O that contribute molecular oxygen to the atmosphere, which recombines to O2 and O3. Arguably, surface activity of minerals exposed to UV can catalyse additional O3 production on the surfaces of dust grains.