Some well thought out points, I just want to add some of my own thoughts.
Having additional droid brains exclusively for higher end guilds would just further tip the balance away from smaller and/or newer accounts even more than TW rewards already do. I like your idea about having them as rewards for a TB bonus planet, but why not just add 2 grydas and droid brains to Zeffo or Mandalore? Both require investment to get a legendary, and Zeffo is still a tricky unlock. 4 of each per month wouldn't upset the balance and flood us with gear, but it would reward the guild wide effort.