I have always had issue with mating parts into a form that creates a triangle. I can get it to work usually, but it kills Solidwork's speed and usually over defines my assembly. I understand why - the program thinks the mates are circular logic. I know I could put my existing assembly into a new assembly and add my supports there, but I want to avoid doing that as it really impedes my design flow. Another solution would be forcing the diagonal supports to solve after every other mate has solved. Perhaps by designating a mate folder to be last in the solve order. Is there a feature like this in the current version of Solidworks? If not, it'd be really nice to have.
The picture is the particular design I'm working on. It's all one assembly with a single weldment part driven by the externally referenced joints.