I took the soundboard down to 2.4mm in the end as at that stage it had a nice ring when tapped. I glued the two halves together and now they need 24 hours for the glue to harden properly.
I already have a pair of sides left over from a couple of years ago and I have decided to use them for this guitar. I need to trim them down a bit as I want this guitar to be a slim guitar. I am contemplating how to cut them as that is normally done before bending them.