Like I said, I'd use 2" EMT. Cheap and strong. A 6x6 is a little overkill. You could get away with a 4x4 as well. Problem with both of those is that they'll rot at the base eventually, even "treated" or CCA lumber. The tree, no matter where you mount the camera, will continue to grow and bury the straps and mount eventually if you don't get up there and adjust things regularly. I have fence posts, 4x4s, buried about three feet with concrete. Yes, they, too, rot at the base.
I've used 1-1/4" EMT for antenna masts for a long time, ham radio operator for years. If that, 1-1/4", will handle a three or four square foot surface area antenna in 75mph winds, a 2" will more than handle a couple of cameras.