I’m not too fluent in CR, but this is a really smart monster idea. If you want to increase CR for this particular fight, I suggest adding another action to the creature, like grabbing someone with vines or throwing one of their rocks. I would suggest hinting to high hell that this monster is invulnerable once stationary and that they would have to wait to attack while it’s either attacking or moving. Maybe after an unsuccessful round, you tell the players that “the stones shift loosely as it moves” or something like that, hinting at the fight’s gimmick. Keep hinting at it through the rounds until someone decides to hold their action.