What about removing the drywall from the ceiling joists, filling with sound absorptive insulation, and then using acoustic cloth across that. Seems to me that you are very limited in height dimension, so it would be better to build your treatment into your ceiling rather than onto your ceiling.
Would this room work?
My wife is falling in love with a house that has a room over the garage 13 feet by 17 feet, ceiling height 8 feet at peak with sloped side walls to 4 foot wide peak which runs lengthwise. This room could be a dedicated listening room with any room treatments I desired. Does anyone have such a room and does it work well? Issues and challenges?