You're right to be concerned about the usefulness of insulation within the wall/ceiling. It would contribute to keeping the room quiet, which is important for having a good sounding space, but it won't change the behavior of sound within your room. This is the difference between the thread (acoustic treatments) and another thread in this forum - the soundproofing master thread.
There are differences between the products, but the differences among them are much smaller than the differences in implementation. So let's deal first with what goals we might have for improving your space, and then determine what characteristics to look for in products.
The hardest to fix with treatments will be bass, so here's my recommendation without seeing exact details of placement and performance. Make sure the subs are in different portions of the room. Add absorptive treatments to any corners that you can (though it looks like that will be tough in your space). Then make sure they are properly set up for delay and phase at the main listening position. Last, apply some mild EQ to flatten the response at the main listening position.
The left and right mains are pushed into corners by the large screen. I'd need to see anechoic measurements of those speakers to be sure, but I bet they are designe to sit out farther from the walls (probably three feet or more). When that's the case, the nearby walls will reinforce the low-end sensitivity and you end up with an overpowering bass response. EQ may be necessary, but you should also try some thick absorption on the side walls next to the speakers to mitigate the boost.
In the rear half of the room, near the seating, I'm less certain. The diffuse character you get from your surrounds is probably a good choice given the proximity to the seating, but there is probably still some troubling flutter echo between the side walls. A few simple panels to the sides of the seating is probably adequate.
For the corner bass treatments, you want as much material and space as you can sacrifice. If you can get to thicknesses around a foot, you should focus on a low density product, like common batts of insulation. But if you will be limited to less, I'd recommend a rigid rock wool panel about 6 pounds per cubic foot.
For the panels near the mains, I'd recommend the same 6 pound product at four inches thick or six inches if you can manage it.
On the side walls b the seating, a 2" panel of the same is probably a reasonable compromise, given the space constraints. Thicker would be better.
This should be a good start. There are other worthwhile approaches, but this is where I would start. Remember to run you auto calibration after any of these changes.