Have you made any changes since the last post?
Great gear and definately a room with potential but you NEED sound treatment. Doesn't have to be fancy or expensive, but needs to be significant...more than just a couple sheets of thin insulation hanging on the wall. Your room is VERY reflective. Laminate floors, no wall coverings or long heavy curtains. On the plus side your funiture looks like it is very absorptive (and dark colored and comfortable too
In my opinion (which is most definately not professional) adding an extra layer of drywall will do nothing to improve the sound.
If I were you (and I was in your position this time last year) I would experiment by temporarily pinning heavy blankets up around the room and laying a few down on the floor. I think you will find that this absorption makes a huge difference, but be careful because it is possible to over do it.
Once you are satisfied that you are on the right track then you can turn your attention to bass response. You can try moving the subwoofer around, or better yet, place the subwoofer where your main viewing seat is and crawl around the room with bass heavy music playing to determine the smoothest location then stick the sub there. Takes a bit of trial and error but this technique really does work. You can also experiment with bass traps. "Super Chunk" corner traps are cheap and easy. Use 24" mineral wool insulation for 2X6 construction. Cut bats in half then cut each half on the diagonal for a total of 4 triangular pieces which are then stacked in corners. From what I have read you need SERIOUS trapping to make a difference. As the bass traps will also absorp high frequencies you need to factor this in to the overall room scheme or it will be too dead sounding, which is the opposite of what you have now and equaly as bad. Once your blankets and piles of stacked insulation get you where you want to be its just a matter of making it permanent and pretty.
As for ceiling rattles...yup, pot lights are notorious but also have a look at the heating vents.
I chuckled at the dancing GIF you posted out of frustration for the lack of feedback on your thread. You take the time to post, thousands have a look, but only a very few ever leave a single remark, or if they do they leave it in their own thread. As for advice I think often the veterans are sick of posting the same stuff over and over again when so much is available elsewhere on the site. And for those who gain ideas and enjoy other's endeavours but never give a single thing back...well, people will be people I'm afraid. No wonder I have such disdain for most of them.
Good luck with the experimentation and let me know how it works out.