I have a large amount of Starline brass that I use specially for hunting rounds. Everything else gets thrown in a mixed brass pile for plinking. I never really cared if the mixed brass was as accurate or not. It is a matter of organization and consistency makes me operate this way.
You should know if the mixed brass is consistent if you shoot it enough. Maybe your accuracy issues are from other things, but you could try buying a large amount of the same brand of brass to keep things simpler.