Maybe we can plant fields of new coal to be found in a million years ;-)

Apparently we can’t. That worked because there were no bacteria that’d consume the biomass and release CO2 back then.

If its deep enough underground, the conditions favorable to that bacteria may not exist.

I'd disagree with "very bad" being the bottom end of the scale. The cost to plant a tree is not very resource intensive and many trees lost would be made up for a single successful tree. It's almost all upside.

The problem is you haven't actually offset anything, and by the time you find out that you have offset nothing, the carbon you were hoping to sequester is back again.

There's other not good (carbon-wise) uses of trees, like harvesting for firewood, or culling to prevent the spread of diseases like oak wilt or bugs like the emerald ash borer.

Compared to chemically weathering minerals, trees are at best a gamble. Most people aren't stuffing dead ones in really deep holes; the best we can hope for now is they get cut up into quality furniture or building materials that will last decades or more.

There's really very little long-term upside to randomly planting trees and hoping for the best, aside from the fact that trees are really nice things and intrinsically good to have. Just not a super effective carbon sequestration scheme by comparison.