I would make all the panels with 2 sheets of insulation (eg 6" thick total).
You should able to take a measurement with REW, then set the graph to 1/3 octave smoothing - that should give you a general idea of the room at that listening position.
Also, write down all the settings, where you set the mic, the gain, level, etc. So when you do your "after" you have the exact same settings.
When you run the test set your DAW to record also, this will give you a visual waveform to look at which can be more helpful than the graph in REW IMO.
See ideally every tone coming out of that test should be recorded at the exact same volume. So you would have a perfectly flat wave form (and a flat graph in REW).
The peaks and dips show the problem frequencies, which hopefully your traps will compensate for.