Sounds pretty reasonable to me, but I'm not sure about the plywood. Treated plywood is rated for outdoor exposure but not, I think, for ground contact. But, if you fill it with topsoil and then rake and roll that, you'd have a surface that should be OK.

An alternative might be quarry sand. In this area you can buy limestone 'quarry sand' -- the bits left over from rock breaking, minus some of the dust. That sort of product would be less likely to shift under the liner than sand.