The charcoal will contain at most half of the carbon, the other half goes up in smoke. You could do cogeneration with that but now you need a plant nearby or heavy transportation which likely won’t be carbon negative. And the more you process it the more extractive the process is, and the we need something that isn’t extractive.
It's not onerous either, a hectare of trees can be sequestered in a 25m2 pit.
Alternatively you can biochar it and then interr, but more labour and time intensive with the extra steps.