He was waiting for the right time. He was waiting until he got enough intel on the enemy before making his final move. If he spoiled the surprise early on, Breed could have counter-acted accordingly and might even have been able to re-attach the collars and order Law to never use his Devil Fruit again. Law didn't know Breed's full power yet at that time and just playing along. Remember how he only pretended to attack Luffy. Law was wearing ear-plugs and he wasn't under Breed's control at that time. He willingly attacked Luffy pretending to obey.
Law did the same thing on Punk Hazard, where he waited to remove the handcuffs when everyone was caged by Caesar. He didn't want to let his enemy know he was free to move about. He waited until he was out of sight, letting the enemy (Caesar) think he was defeated, giving him a lot of space and time to freely move about preparing his final counter attack.
Finally, I think Law is incapable of actually removing the collars, since he didn't actually removed them himself, even when he shambled to the submarine and had plenty of time to do so.