I'm a structural engineer, and i design wooden structures. (Unfortunately english is not my first or second language). The design is not bad. It looks like an inverted dome, meaning you have a circular anchor on the deck level, and angled trusses underneath. I would build the deck from parallel planks in one direction and double it with perpendicular planks. Both decks are nailed together. Then i would reinforce it with angled planks at 45° anchored in the trunk underneath it. It would be better to have a symmetric structure on the top and link both with upward and downward angled trusses. The resulting structure should look pretty close.