Or maybe it’s just the case that the amount of energy needed to create mass is astronomically minuscule.

It would actually an astronomically large amount. An atomic bomb will turn a very tiny amount of mass into a tremendous amount of energy. And that’s with a nuclear process that is way more efficent then a chemical one like photosynthesis.

But from pure physics standpoint a carbon atom and an O2 molecule will have a teeny-tiny bit less mass then an CO2 molecule (which is why combining or burning them together will release some energy). So doing the reverse and splitting up a CO2 molecule into it’s parts will generate a little bit of mass.