Just thinking about the power gem scenario here, didn't Thor beat Drax when Drax had the PG, and they started out as roughly equals. Odin starts out above Thanos, so I'm not sure Thanos could take Odin even with the Power Gem. At the very least, there's nothing in the OP saying BFR isn't an option either. Or couldn't Odin just teleport the PG out of Thanos's hands?