I got a mace. Ive been playing since mid December and do most of my battles with allies for the extra tokens. It's the only one I've gotten from a bronzium, so I'm guessing somewhere around 1:250
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Got Poggle 4* from a Bronzium. Got Daka shards cause I already had her but would have pulled her. I think that's it and I've opened probably thousands and thousands of them. I would say odds are close to 1 in 1000 and I base that on absolutely no evidence other than saying it's really really low.
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Boba does drop from bronzium. Rex is chromium only.
The bottom row actually does show the full characters you can pull, just scroll through. But here, the 4* chars available are:
Datchha, Urororo'R'R, Boba Fett, Mace Windu, Poggle the Lesser, Ima-Gun Di, Barriss Offee, and Old Daka.
EDIT: I believe I also got Mace and Ima as well. Both 4*