Is my treehouse platform structurally sound?

I’m working on my first treehouse as an adult ? I’ve built plenty out of scraps as a kid, but nothing of this magnitude. I’m mostly concerned about the structural integrity of the platform with this post. I’ll layout my general plan, post some pictures, and ask some pointed questions at the end.

I’m planning an 8x12ft platform where 8x8ft is enclosed with a roof and 4x8ft is a deck with railings. Right now this is built around 2 trees that have a diameter of ~12in and sit ~5.5ft apart. I’m using two long 1/2in stainless steel tubes/bolts that go completely through the trees to hold the main beams. The beams essentially rest on the bolts and sandwich the two trees. On top of those beams the floor joists rest perpendicular. All wood is pressure treated.

Pictures of where I’m at (click to enlarge):

Questions:

• As you might be able to see, everything is currently built using 2x6in boards. I think the floor joists will be fine; I’m not dry walling or installing a fridge ? That said, I’m second guessing the beams. Should I double up the 2x6in beams or swap them for 2x8s or even 2x10s? I’m leaning towards doubling up the 2x6s since they’re sitting on top of the bolts and I have the full 6in of vertical support.
• Obviously I need to add some knee braces. My initial plan was to run 2 up on the outside of each tree parallel to the joists for a total of 4. However, I’m thinking maybe I should shift those 4 to the inside of the trees and then run 4 more out to each corner for a total of 8. Thoughts?
• Last question, am I missing anything else obvious? ?