Firstly I should point out I have only a little experience in board building. But what experience I do have may be relevant to your project.
I have built a 120 x 50cm board using a 20mm core.
Even though the majority of the strength comes from the skins, I think the core material still comes into play both from a compressibilty and shear strength point of view. I used a 75kg/m3 closed cell PVC foam (quite a bit tougher than the standard EPS).
The main layup consisted of:
200 g/m glass
200 g/m glass
200 g/m carbon
200 g/m carbon patch strut mount reinforcement (maybe 20cm x 40cm)
core
200 g/m carbon patch strut mount reinforcement (maybe 20cm x 40cm)
200 g/m carbon
200 g/m glass
200 g/m glass
Top surface was done in one shot with all layers wrapped around rails (vac bagged)
Then bottom surface was done in one shot with all layers wrapped around rails (vac bagged)
All layers were standard woven twill.
Also included were a couple of wooden (western red cedar) stringers (600mm x 12mm x 20mm) either side of the strut mount
The result was unbelievably stiff. I could support the ends and bounce around the middle with little flex or fear of breakage.
In fact it was surprisingly stiff with just to top laminates applied.
I made a bit of a mess wrapping the rails. This needed quite a bit of sanding and i ended up covering these with another layer of wove glass tape (200g/m) but this was more for aesthetics and ding protection than strength.
Maybe relevant, the strut mount is a Ketos 'KF box middle' which was bonded to both top and bottom skins.
In my view the carbon provides most of the stiffness and the glass is to protect the carbon.
The board came out at about 2.8kg painted.
I've only had a few sessions on it but some of these have been struggling to get out through breaking waves in an onshore wind....lots of groundings and board tumbling in on the surf. A bit of a baptism of fire for a new build !!
All has held up fine so far.
I think it is more than stiff enough. Only time will tell if is strong enough (I'm a bit paranoid about loosing my foil !)
My thoughts for the next one are:
1) Don't bother wrapping carbon round the rails - I don't think its needed and its difficult to do well.
2) Probably only one layer of glass on top surface - most of this is covered in deckpad anyhow.
3) Maybe eliminate stringers in favor of an extra carbon patch top and bottom.
4) Plate mount - oversize holes filled with epoxy glass, prior to lamination, then drilled to size after lamination.
Hope this helps?