If you are using the X-mount that comes with the motor then I think a 1/4" birch ply is the way to go. Also much easier to integrate into the wooden fuselage structure.
If you are front mounting the motor without an X-plate then the hard nose is a good hard "forever" mounting surface, and the G10 can be prepped to be captured by epoxy. However even with a front mount, I have made several successful installations using 1/4" ply, but I also line the mounting holes with brass or alum tubing to help resist crushing. Again it is much easier to integrate the wood firewall into the front end structure.