I'm glad the feedback helped. I enjoyed reading through your thoughts. If you are still thinking about the Rock Throw ability how about this alternative:
Four-Arm Smash. With your Bullier you can use your action to smash the ground with all four fists. When the Bullier smashes the ground, pick a point up to 60’ away. Each creature within a 10’ radius of the chosen point must make a Dexterity saving throw. The DC for this saving throw equals 8 + your Strength modifier + your proficiency bonus. A creature takes 3d6 bludgeoning damage on a failed save, and half as much damage on a successful one. The damage increases to 4d6 at 5th level, 5d6 at 11th level, and 6d6 at 17th level. After you use your four-arm smash, you can't use it again until you complete a short or long rest.
That way you don't need to have it be dependent on too many rocks being around or not enough. The mechanics stay the same. I added the bludgeoning damage because you didn’t indicate a type.
Four-Arm Smash. With your Bullier you can use your action to smash the ground with all four fists. When the Bullier smashes the ground, pick a point up to 60’ away. Each creature within a 10’ radius of the chosen point must make a Dexterity saving throw. The DC for this saving throw equals 8 + your Strength modifier + your proficiency bonus. A creature takes 3d6 bludgeoning damage on a failed save, and half as much damage on a successful one. The damage increases to 4d6 at 5th level, 5d6 at 11th level, and 6d6 at 17th level. After you use your four-arm smash, you can't use it again until you complete a short or long rest.
That way you don't need to have it be dependent on too many rocks being around or not enough. The mechanics stay the same. I added the bludgeoning damage because you didn’t indicate a type.