For these large drivers, you need a pretty thick front baffle if you flush mount. I've used 1" MDF for "thin" drivers or double 3/4" for larger drivers on the baffle. Once you route out the recess for flush mount, you're still left with a good 3/4" of material to attach the driver.

3/4" is plenty if you use T-nuts and bolts. More would be better if using wood screws. As for your 5mm, I'd see that as risky. Remember, you're looking to do more than just hold the driver in place. You want to screw the thing in tight so there's no air leakage and that can put a lot of pressure on the material behind the driver.

Mind you, you don't have to re-enforce the entire baffle. Just the ring around the driver will do. Above photo is my driver re-enforcement, front baffle bracing combo.

Gar.