How about we grow corn (or other fast growing biomass like kelp) and bury it?
This is most easily done in forest, but note that you need regular forest fires, otherwise the fire is hot enough to run all the carbon into CO2 and we didn't gain anything. Thus most of the forest fires you read about are bad - but if we change our policy after the fire and burn those forests often we can make them a net negative CO2.
Carbon emitted from the decay is not net positive into the air. It would still be net negative so long as you didn’t expend fossil fuels to bury it. In fact, I could burn some of the harvest to fuel burying the rest :)
Plus burning it would release the carbon faster than decay, and decay of CO2 into roots is generally just fixes by growing plants again. So the rate of carbon release would be dramatically faster to but versus decay. And what we want is net sequestering so slow release is good.
It’s literally the process that generated a low CO2 high O2 atmosphere in the first place, I’m just proposing we cultivate it and not harvest.
But I suspect we'll need a lot of forests to compensate for the current rate of fossil-fuel use.
Secondary problem would be handling all of that junk. You cannot burn it, have to bury it instead.
What do you mean by "you can't burn it"? It would release some amount of CO2, but as another commenter points out, there would still be a non-zero amount of left over carbon, no?
If you want to go full genetic engineered plants, there are better options out there.