I agree that a living tree is not much of an offset. Trees are short term (and small) carbon stores when compared to fossil fuels. However I do not think that burying a tree solves the problem. Unless we find a way to prevent fungus from breaking down lignin, I am not sure that burying a tree is a viable long solution for carbon sequestration. It is outside of my area of expertise, but I can find at least one paper that suggests that coal deposits formed before the evolution of fungal species that could break down lignin (https://www.science.org/doi/10.1126/science.1221748). It is extremely difficult to turn CO2 back into coal. I am in favor of reducing forest loss (and ideally increasing forest area), but I am unconvinced that it will have any meaningful effect on atmospheric carbon in the near, mid, or long term.
There's some research that shows that burying logs super deep (beneath the biologically active layer) is an effective method of long-term sequestration: https://pubmed.ncbi.nlm.nih.gov/35362755/
Well, not a mature forest, but one of those "tree farms" of evenly spaced monoculture sounds like it'd be a great candidate.
Of course, since those tree farms are explicitly for production and therefore owned by the owner class, who don't really want anything to do with this whole unprofitable "saving the planet" thing, the above scenario is more likely -- but all I meant to suggest is that it'd be possible to use this method and not destroy precious biodiversity at the same time.