I don't see anything wrong with the file you sent. Since the braces are all fixed-ended, they are resisting some moments, in addition to axial forces. One consequence of making the sections larger double angles is that they will attract larger forces relative to the other members (larger moments in particular), and this could ultimately result in a higher interaction ratio.
For some tips on single angle design, note this saved search: communities.bentley.com/search
For some tips on single angle design, note this saved search: communities.bentley.com/search