Soundproofing in drywall partitions

Since I am getting an exposed roof truss, some rooms adjacent to/in the hallway above the normal room height are still unfinished or open. I will then extend the regular brick wall with drywall up to the roof ridge to close off the room and completely separate the rooms from each other. If I only use one drywall panel, that would be visually completely sufficient, but in terms of soundproofing, it is inadequate. Especially since the bathroom/toilet area is within hearing range of the living/dining area, and I do not want bathroom noises in the other rooms, and conversely, I do not want disturbing noises from the other rooms in the bathroom (for example, hearing the TV while bathing). It certainly wouldn’t be a problem to build a double drywall up to the roof ridge and fill the cavity with a material that insulates sound. I just don't know what would be suitable for that. I was even thinking about sticking those big egg cartons between the drywall sheets :) A masonry wall up to the roof ridge is actually out of the question or rather I want to avoid it as much as possible. For better understanding, I have two sketches, one from above and one from the side as if you were standing in front of one of the mentioned walls. In the top view, the red lines mean that this area above the normal brick wall is currently still open.