For me, I first offer Bradley, filler and picks. Which will be denied. My second offer consists of Bradley, Bass, and picks. If the prefer Lee and Bass even better but I think Bradley will be a top a lot of GMs wishlists if they are dealing with the Celtics.
Your problem is that the players with contracts that can be used as filler are Terry, Lee, and Bass if you want to get someone back who has at last a MLE-sized contract.
I disagree about Sullinger having a higher ceiling than Bradley. If you are saying that most GMs will value Bradley more than Sullinger, then I agree.