That's a low ceiling! I assume when you are talking about the interior and exterior drywall, you are NOT having a room within a room, but are just cladding the external surface of the single stud with the one layer, and then putting the two layers onto the inner surface using the resilient bar? My experience of these is that they actually don't do very much at reducing the low frequency energy at all! Reduces speech, but if you thump on the inner surface, on the other side of the wall, you can hear and feel the thump. I've done I think 8 studios now, and all were timber framed bar one. Steel studwork is nice and quick to work with but I'm not convinced at the sound reduction capability. You might think about scrapping the exterior layer and adding it to the inner? That would make 3 layers which with staggered joints is better. From experience - weak points will be where the roof layer meets the wall - I developed a rather simple cheat with my timber studios in the later ones. I would build the walls and NOT put in the ceiling joists at all - so, the timber studwork and the sheet cladding. Then I would sit 2 12mm layers of drywall (plasterboard in the UK) sitting on top - so totally sealed at the top plate. Then I put the joists on top and secured them trhough the plasterboard. Downside is that it means you cannot screw the usual way through the wall timber, into the joists. I discovered this did not matter, because the weight is considerable and gravity and friction did not need physical screws. Once the entire ceiling is done, working in from both ends, the middle can be finished with some cutting and tweaking. Whatever you put on the walls is limited by the layers on the ceiling. With your low ceiling, I'm guessing your joist height will be compromised - so weight might be tricky - the deflection downwards from the plasterboard weight can be quite worrying if you are compromised in the joists. Possibly with metal studwork, you can just double up the channel to give extra vertical load bearing capacity?
If your insulation will be an issue without the external layer, you can always use chicken wire as a backing and have the insulation contained in the stud channel depth. In one studio, one room had no insulation at all - and the same depth of sheeting as the room next door that did have the insulation. If I am to be honest, we did not notice a difference. Thick plasterboard simply does not let enough sound through to make the presence or absence of insulation very obvious - if at all? I don't think we even detected any heating differences in cold weather. Both rooms seemed identical. One tip is that spending lots of money on extra layers is usually a bit pointless, because it's always the weakest link that does the damage. Ventilation holes, oversize holes made by the electricians, and in one case a piece of conduit installed through multiple layers of a room within a room to get a mineral insulated cable inside for a fire alarm buggered up all the effort we put into insulation. I remember that one well! We started with a soundproof room even with the crazy metal drummer, then suddenly people in the space next door started moaning about drum noise - that bit of plastic pipe ruined it. An air gap from inside to outside.
I'll be mainly using it to record drums and guitars and vocals (separately) as well as some minor production with mainly iems
