This is a little late but you have used your strongest material in the area requiring the least strength, i.e, the very bottom.
The beefy and no doubt expensive heavy wall tubing would make excellent struts angled from closer to the top of the column to the ground,
where they could then be tied to the center column with cheap flat stock which would only ever be under tension. For the same cash outlay you
would now have nearly indestructible trusses supporting your load. What you built is similar to a table base with the weight kept low for ballast.
I'll admit that it is difficult to accurately gauge the thickness of your tubes from the photos, so apologies if my statements about them are in error.
While I'm at it, thin wall may not be the best choice for the center column but would probably be OK if it was supported by the angled heavy wall.
As you have done it though, all the loads converge where you have attached the base tubes, weakening an already somewhat weak material.
Prices being what they are, I would have used the same center tube size for the whole structure, increasing strength and probably saving money.
