Re lay-up: The original Granda Plata – which is similar in size and shape (though it has no concave) – used 11-ply baltic birch. No composites. It’s working well. I have about 1300 km on mine. The deck is holding up. I’m having minor issues with the enclosure. It cracked at the corner attachment points, which I think might be caused by the deck flex to some extent. So maybe adding some composite plys to make it stiffer would be good.
I’m by far not an expert though and you definitely have more experience so best if you go with your own judgement.