Marcos Scriven wrote:

The angle brackets seem very secure at the wall, and there dooesn't
seem to be a problem supporting the weight. However, the brackets
themselves bend a bit, especially when playing loud cords. I've
thought about attaching legs to the ends of the brackets, but I'd
prefer to avoid that.

It sounds to me as though the wall, and your fixings to it
are perfectly adequate, it's just that the brackets aren't
stiff enough.

Try and find some with a built in diagonal bracing, like this:


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Nick