durarock or cementious board would be the thing or flooring of choice, but your ply wood being that thick should work BUT. I would suggest that you screw that ply down with non rusting 2 inch screws every 6 inches to ensure any warpage does not occur. Nails will NOT work. Need screws.
If you screw it down thoroughly and sink the heads below surface, the moisture in the thin set should not cause any warpage.