I think you'll find that if the cartridge cases had been non-combustible they'd actually been a fraction smaller than the combustible, cardboard cases (as metal is stronger and hence can be made thinner).
I also think you'd have found fitting a muzzle-brake to the gun difficult as the fins from the missile would have been likely to foul it. It also wouldn't have made much difference. The reality of the Sheridan was that the 152mm gun was simply too large for such a light chassis. When the Australian Army trialled the vehicle they remarked quite unfavourably on its massive recoil noting that it tended to leap a considerable distance backwards when fired, even with the handbrake applied hard on. I've often wondered if the vehicle would roll on its side if it was fired athwartships, rather than fore-aft as it seemed to be.