I'm not sure that would scale. According to their website (https://www.draxbiomass.com/), each plant produces about 1/2 metric tonne of wood pellets / year. The Drax plant uses 7 million tonnes / year, so you would need 14 equivalent plants to produce enough wood pellets for just one power plant.

In the 1980's, old growth logging was shut down in my state (Oregon) for environmental reasons. The new growth forests had a long growth cycle, so the two big operators (Georgia Pacific and Louisiana Pacific) moved the bulk of their operations back to the South, where the Southern Pine had a 12 year growth cycle.

I think a reasonable guesstimate would be that the max wood pellet production from renewable forest land would be far less than the input required to convert all the existing coal-fired plants to biomass. It's just one play in what will have to be a multi-faceted approach to green energy.