Right so your project cost is right up there with an incredibly expensive discount surly. Big deal. Also I found some tubes at about twice the price. Not sure there is enough stout stuff yet for what I want at a price I want it for...

"This means that a circle, where all the edges are the same distance, is the stongest."

It's really an efficiency issue. anything can be strong if your materials budget is big enough. What the tube does is gives you the best deal when the load can come from a lot of different directions, Like a fishing rod, where you can wiggle the thing in almost any direction, and when you do the down and dirty to land big fish you are loading it in all kinds of directions, so a tube is a good choice. I guess another example is bamboo. wind probably comes in all directions even if it predominates in one. Now a bow is not a good structure for a tube because the string determines that all the load comes from a narrow range of angles.

"Triangles are also very strong, being used primarily in bridges, and you can see (two) triangles on bikes, which is where the strength in several directions comes from"

Triangles are great because they support each other in such a way that they don't shift the corner angles. The triangles you see in bridges keep the top and bottom trush cords appart in such a way that the truss comes very near having the same strength as if it were solid, but with a fraction of the material.