Forgive my ignorance, but why is excess CO2 such a challenge? Isn't it extremely easy to simply vent it into space/the nearly-empty moon atmosphere?
You need about 2000 m2 of corn to feed one person over 1 year and just 100 m2 of corn will capture the CO2 exhaled by that 1 person in 1 year. Assume we live in horizontal lava tubes 300m wide, let's say 10 meters of that tube per person to have some margin of error. That's <535 kg of CO2 in air in that section of the tube and ~8000 kg of CO2 captured in corn planted there the moment before harvest.
It's doable IMHO. At least napkin math checks out.
Get plants with a more favorable Edible Carbon::Total Carbon ratio.
A plant that can be harvested continuously would also be beneficial.
The ideal plant may be something simple like grass. We can't eat it, so would need another intermediate step. Possibly Cows or Sheep. These would be grown for milk and not meat.