1x8 roof sheathing VS plywood/osb
If it were my house, I would leave the blown in insulation in there and remove enough sheathing to blow in more, say maybe 16" total or at least dd enough to get it to code.

I would think 1 or 2 boards about every 6-8' apart will allow good coverage and depending on the roofing going back down, just add a layer of plywood over the existing sheathing since it is a strong base there to build on.

If the space is a continuous 20" deep over the whole attic, only place a maximum of 18" for the hot air to travel over the insulation to the vents