Standalone Drum Room Construction

Hey guys, greetings from sunny Brisbane. I'm currently in the research/planning stage of an upcoming build, I have a few questions and also looking for any advice/ideas. The current plan is for a standalone room within my backyard tin/colourbond shed. Primarily its main function will be for a drum/band practise room, though I am intending to move all my studio gear in and use it as a single room studio down the track. After taking some levels at a recent rehearsal I was hitting around 120dBc with 125dB being the maximum. Im hoping I can get around 70dB reduction from the room alone. About the room and shed. The shed is a 9m x 6m tin shed, on its own slab, about 20meters to the nearest neighbour. It has a vaulted roof, with a height of 2.9m at the peak (2.7 at the bottom of the rafters), and the entire inside ceiling is outfitted with some form of 20mm thick high density foam panel I can only assume is used for heat insulation. The room I am hoping on building will have outside dimensions of 4.5m x 4m and inside footprint of 4.1x3.6m or thereabouts, and as much height as possible. My original plan was to build a fully standalone room within the shed, using a double stud wall design exactly as in the picture attached, though with green glue between the inner 2 layers of gyprock. From inside to out walls will be consisting of- 10 & 13mm gyprock with greenglue between -> 75mm frame with earthwool insulation -> 25mm to 50mm airgap -> 75mm frame with earthwool insulation -> 10 & 13mm gyprock. I was planning on either sealing them directly to the slab, or have them sitting on neoprene pads if neccersary and then sealing. Thats about where my plan ends, I don't know how to make the ceiling work. Now I have priced up and budgeted to use the same design and materials (minus green glue) for the ceiling, but I just don't know how to physically build it while still maintaining the isolation of the double frame, and also a safe and sturdy room. Apart from not knowing how to do the outer leaf of the ceiling, I am also concerned about the existing insulation panels acting as a third leaf. After using the MSM TL calculator I found on here, Im fairly confident the wall design will do what I need, but the ceiling really has me stumped. I am basically just looking for suggestions and advice on how to achieve my goals. Also my current budget is 5k, though there is a little bit of movement if needed. Hopefully this all makes sense, Thanks in advance. Rory.