Step 9: Brace the platform
As it is, the platform will wobble dangerously. Add diagonal bracing made of 2×4, and use a single long lag screw to attach both of these to the tree. It's easiest to just cut the 45 degree angle in the 2×4 first. Use an 10" lag screw here to make up for the fact that you're going through 2 thicknesses of lumber.
Note that I just used one set of braces on the single tree, because the other end had two trees and the wobble seemed insignificant. You'll need two sets for sure if you have just a pair of trees.