Hi Glen, I'm glad you're taking this direction . Not only should you have a better "sounding" floor you also address many of the potential issues outlined earlier.
I'm guessing from your OP and the quoted figure of ~2m3 of concrete your slab will be 150mm thick? That's exactly what I'd recommend as at ~5m (taken from your stated 16' length) your on the limits of what concrete can do without cracking.
If your not thinking about laying the concrete on Polystyrene (not very expensive and available at most builders merchants https://www.insulation4less.co.uk/eps-70-100mm ) I suggest running a perimeter of 50mm Polystyrene to the external walls. You can use this as part of the shutter for the concrete which will give you something to tamp off, ensuing your slab is as level as possible and can provide the foundation for a rudimentary cavity tray. If that doesn't make sense I can provide a basic sketch to illustrate.
What you have planned is what is required to pass residential sound testing on party walls. I prefer the idea of using PLY or OSB but completely understand the cost implication. I would prefer this as using different materials will significantly reduce vibration transfer. I have no alternative to Green Glue other than the aforementioned, personally I'd be a bit more creative.After that's laid, I can start on building the actual walls etc. I'm also now going for double plasterboard. Because of width and cost constraints, I'm using 9.5mm standard, with 12.5 Acoustic on top, for the walls and ceiling. Hopefully that'll do the trick.. enough for my needs anyway.
Cheers
I would consider using cork / vinyl / rubber sheeting. This is pure blue sky thinking on my behalf so I have no examples to share but, I know ~3mm cork is cheap and very effective for Slate/Cork/Bamboo isolation platforms, vinyl could be picked up at low cost in the form of vinyl flooring remnants and I believe neoprene is ready available and low cost. Just some random thoughts and based on nothing other than my own imagination so take it with a pinch of salt. I do however feel that any damping achieved would have little effect within the listening room but more outside of it, so probably wouldn't invest too much time or money into the endeavour, I'd rather spend it on more insulation!