We had a big wind storm come through earlier and test the strength of our canopy. As you guessed, one length of twine broke, though the rest held. The big issue turned out to be the bungie cords holding the tarp to the metal support structure, as quite a few of them snapped. No damage to the posts or the tarp itself though.
As a result, we've replaced both the twine and the bungie cords with nylon rope. Time will tell, but I anticipate that both will hold much better.
I do not understand, why not just extend the legs to the concrete driveway and anchor. I would also install a couple of stabilizers from house to upper horizontal post.
How should we extend the legs? The canopy itself is only 6 feet tall at the edges, which necessitated the blocks. And I want to be able to remove it quickly and easily if it doesn't work out, so I'd rather not connect it to my wall.