The added weight is distributed over a large area, so that ought to be of little concern.

This following is not a professional recommendation: However, if it were my house and my job, this is what I would do...

Since the laminate is a floating installation, all you need is a reasonably dense and flat panel to raise the level to where you want it. Almost anything that will lay flat will do that. Perhaps extruded polystyrene insulation panels (XPS) would work. The panels are light in weight, dense enough, won't rot and will lay flat over the existing plywood. Tape them together and you've got your flat smooth substrate.

As long as your existing plywood floor is dead flat...or at a minimum flat to within the laminate manufacturer's recommended degree of flatness, then any flat panel laid down over the existing plywood will also be flat.

Again, not a professional recommendation, but the (XPS) panel is what I'd consider using. You can get XPS in 1 inch thickness.