Random comments some may or may not be useful....
I like to make my boards LIGHT....
A board with low inertia is the tits. Require less wind to stay on your feet, less effort to turn and physics says it'll stick to your feet better than others over the bumps and such....I guess your making one with straps though......still low inertia is the best.
I dislike carbon unless very carefully located as it has poor impact properties and adds stiffness which most builds that are tough enough to take a landing don't need at all. the rails isn't a horrible place for it and won't add a lot of additional stiffness still it doesn't really gain anything much beyond the sexy bling and you lose toughness. It might give you some good "spring back" but it's unlikely the board will have enough flex or a natural frequency where the spring back will be useful and still be able to take a good landing.
I've found if you get the rail too thin the board starts to bury it's edges a little too deeply on heavy carves. Sinking the entire front of your board on a deep carve ends your flow on the wave but increases your flow in the air pretty fast
the more layers of anything often times the more wasted epoxy you have just gluing shit together. Foams and most other things tend to suck up a rather offensive amount of resin or resin and micro when you glue em together.
right now I'm using 1lb EPS in my builds with a heavily glassed corecell sandwhich and can slam out a 5-6 lb finished board.
I'm still determined to do better especially on the flex front.
I'm not hitting the flex I want without getting excessively low on rail volume.
I'll know a lot more in the upcoming months as I've build some shit to do some more rigorous testing.
As to stringers they were originally added to PU/PE boards with a 2 x 4oz top and 4 oz bottom on 3lb foam to get some additional stiffness spring back and strength to a tissue paper lamination On most boards that can take a landing you want to decrease stiffness....I don't see much purpose to the stringer on most kiteable constructions.
EVA foam (foot pads) haves a massive impact on toughness as they can absorb and disperse energy better than a composite fiber that is loaded off axis. (jumped on)
Also important the % elongation of your resin exceeds that of your fiber in order to make the most out of the structure in off axis impacts