How slowly though? Intuitively, I would think that trees piled wide and 20m high and covered over would still be mostly there in my lifetime. Is this wrong? (genuine question)

500m high is another level. If it took, say, 1000 years for them to decompose fully and release their CO2 in to the atmosphere, that's still potentially useful even if its technically net zero.

We really only need to get past the next 100 years.