My current stands are pretty close to your design; 4x4 posts, 2x4 "joists", and 2x2s comprise the top. I don't use any support braces for stands that hold one tree. After 8 years untreated, they're still going strong, but I've had to replace screws here and there.
I am using 6x6 posts for the new garden, just for aesthetics. I have 3 more posts and 2 benches left to place. I'm also planning to place a post somewhere with a solid background so I'll have a permanent photographing spot. I will miss the fence backdrop, but I think the trees will be better with a little more air circulation.