Thanks a lot for the tips and suggestions
This is my second board. The first one seemed to work fine, and it had no carbon and just a single layer of 6oz glass top and bottom. It was about 20mm thick. It seems easily stiff enough, but I only use it with a freeride kite foil (stringy foil).
This time I've made it quite a bit thicker, about 25-26mm, though it's not completely solid. It feels stiff, quite a bit stiffer than the last one was before I glassed it. But this time I intend to use it with a surf foil, so it does need to be stiffer and stronger, especially front-to-back. So I've added the strips of 200g uni carbon in this direction from the back mount holes to where my front foot will go. I'm only relying on the fibreglass for lateral stiffness and strength. It won't have too much effect front-to-back as the carbon will take up all this load.
I'm definitely going to beef up the underside a bit, and put probably 3 layers over the base plate, extending in the area in front of the base plate, which I've found is the point of maximum stress. I am contemplating putting some carbon there, but it might not look that great.
I'll laminate the other side and see how I feel after that - maybe I'm just being lazy