Carry a piece of plywood to one of the far corners of the attic and lay it on top of the installed boards.
Reasons to put plywood in attic.
The decision regarding the thickness of plywood to use for the subfloor in the attic has a lot to do with how you intend to use the attic and it also depends on the spacing of the ceiling joists.
Stop using your attic for storage.
So you can put 8 inches of insulation down then put the plywood over that.
Because the simplest and cheapest way to insulate an attic is to add material to the floor.
Plan to pull up the flooring and layer new insulation on top of the old.
But if the floor is covered in plywood you can t stuff enough insulation beneath it to do the job sufficiently not even in warm climates.
Maneuvering heavy 4 x 8 sheets of plywood or osb oriented strand board up to an unfinished attic is sometimes like trying to fit a square peg into a round hole.
Now in your particular part of the country you re going to want a lot more insulation than that.
So what you might want to do is sort of map out an area in the center of the attic where you have your storage and then the rest of the area you could put more insulation.