I'm right in the middle of this at my studio and here is what I'll offer for advice. First I figured out which frequencies are the most problematic. If your room is a normal box shape, John Brandt has a great room mode calculator that will generate a lot of great information to start with. acoustic tools Also consider using a tool like REW (Room EQ Wizard) with a cheap omni measuring mic to actually get data on what your room is doing.
In dealing with room modes, it is important to understand how many of them are related. For instance, my room is 12.5ft wide which is right the exact distance of the 90hz wavelength but also a fundamental of 180hz and 360hz and so forth. In my room the Schroeder frequency between pressure and flow modes is 118hz. Understanding how to deal with troublesome pressure modes is often a challenge where absorptive treatments often comes up short. The good news is that taking care of the 90hz problem helps with the other fundamentals.
I ended up with 2ft deep of absorptive traps along the entire rear wall and 2ft square in the front corners. It is a lot of cubic ft of absorption yet I'm still working out that 90hz issue from the side which will likely take limp membrane traps to resolve.
So all that said, a few inches of rigid fiberglass, foam or whatever will not do much below 200hz despite what anyone tells you. It becomes a matter of ratio of absorptive material to existing space and that ratio is usually too small to be effective.
Let me know if you'd like some help sorting this all out if the room mode calculator I linked is confusing.
What I had previously was 9 inches of 703 (knauf equivalent) in the corners. Here is a graph comparing low density R38 12 inch x 2 (brand pink) on the blue line vs the 9 inches of 703. Maybe you're thinking, why not just go heavier with 703? At higher density, the gains flatline and eventually low frequencies see the absorber as reflective which is the opposite of what you want. Also I have an attic studio so only knee walls. Giving up that short 4ft high space isn't much of a ask. Going fully across the back was only an additional 4 ft. Add my sub to the equation and I needed absorption that went lower. Last I'll note ratio of absorption to volume of space. I needed more.You used 2 feet of WHAT? on the back wall?
It is just a mock up while I work out the remodel and new floorplan. The boards on top are 703 panels just to keep the insulation from flopping over. Going to fill the middle where I've got the old red 703 panels leaning. Got some R30 to cut into wedges to continue up and will then just picture frame the entire wall end to end with dacron and fabric. It is not a very useable space except for guitar cases but did I mention I have a lot of instrument cases. Probably build a rack for them somewhere. Odd note: If you are going to do audio measurements, it is a good idea to remove the most resonant instruments. I had 3 banjos and a pandeiro in the room making a racket with the sweeps.Oh, for sure - knee walls, and hence the sloped ceilings, would be a killer. You should have your pink stuff right up to the slope though, rather than wood on top, as that's creating another hard corner.