Wow!
Several things I noticed. I'm worried about the size of the room once finished. The closer together the walls are, the deader you need the room to be. Second, I'll tell you about a great vocal booth I built in a room about 12'x20'. (It had two layers of sound stop on the walls and then we stapled muslin drop cloths onto the walls.)We constructed two 4x8' panels made of dense construction styrofoam with sound stop covered in muslin on either side and a 3'x4' double panel plexiglass window at one end. We hinged these, covered the hinged joints with dense rubber flaps applied with the panels at 90 degrees so they would flex as your set the angles. We pushed that up against a wall and angled the other panel to allow the singer to move in and out. We could always close the panel completely if necessary. That made a faux isolation vocal booth that would set back into the corner hinged at 90 degrees if we needed more room. We threw a large panel of styrofoam on top during recording. I love your dedication and your efforts in actually building a room. Remember, though. To your wife, that's always going to look like her new sewing room, LOL
Rod Norman