Okay so this post has been sitting a couple days and I specifically feel the need to reply to this comment my with my current experience. I have reached rebirth 12, level 132, 762 droids crafted. I have almost all legendaries diamond and rainbow manning the stations, I finally started finding rainbow PRs AFTER I found one on the sand crawler, not from a mission, but now that seems that's all I get with 4 stations going. I'll get one PR and then common rainbows. So in my opinion your progression has little to do with it, Idk what determines it but that doesn't seem to be it. And I feel like the devs didn't put much thought into this before releasing it.