forgive me, if i have misunderstood you construction methods.
All of your boards appear to have plywood on the bottom and foam on the top ( with perhaps a bit of glass on top of the foam)
But you need to locate it between layers of strong stuff. specifically, the top layer of reinforcement should be as strong (or stronger) than the bottom skin.
If kite-stuff.org wasn't down for a couple of months now, I would simply give you a link from my report of my first 4 boards I did at the same time, but basicly that's the story. Ply for the bottom (2 X 2,5 mm), foam on the top (8-12 mm), carbon pieces under heels and everything laminated with glass.
There are a lot of how-to's, 100's of different constructions and everything ''works''. Everyone reports just about driving characteristics, but usually no one gives any feed back on long term durability of the construction.
My intention with first 4 boards was to experiment. For instance how does the vacuum bag wrap different edges (sharp, chamfered, rounded), how would x-laminating the plys cut at different angle affect the flex, different core thickness, different shapes and sizes. And learned a lot!
But I didn’t really think about construction failure. Obviously I am over-confident
You can see the second construction, if you look at first picture I posted in this tread. I replaced the foam with poplar ply, but not completely. There is still 6mm under the ply, and around the edge of this upper part of the core and around the second bottom layer ply, which is smaller (for progressive flex). This two foam edges are for cosmetic purposes only. And are the main f***-ups in this construction, because in the middle of the board this foam goes from the edge to upper ply (about 30 mm) and the glass has only this crappy material to bond to in the area which flexes a lot. And here the delamination happened (15 cm wide), and I suspect the upper core layer also delaminated in that area, because concave is most distinctive there and the upper ply wants to be flat (if you know what I mean!?).
Now I am replacing foam edges with wooden ones and routed the core foam layer for 15 mm and applied 30 mm wood stripes (15 mm come out of poplar core, to sand it down as previous) and I also applied 2 20mm stripes longitudinal and 1 across (in the middle). So there is direct wood to wood connection between the upper and bottom core parts, but the board looks the same. Hopefully it won’t be to stiff now. Foam is now just a filler for middle core layer.
I wish this board will last at least one year
Now I will do extensive tests on this one, before I start to modify the old ones if the construction turns out to be OK.