I would not add a wall reducing the limited space you have. It is entirely possible to treat the room and have superb acoustical performance, even if the room is a perfect cube. The idea of building a new wall to provide non parallel walls I would also dismiss. Having a changing dimension will not remove problematic nodes, just change them for different ones that still need taming.
You have what can be considered a small room and as such needs absorption more than dispersion. Absorption must take the form of broad-band construction to be effective. The often recommended: 'just add a rug/curtain/bookcase etc. are only narrow-band absorbers and although they are better than nothing they will not reduce the decay sufficiently across a broad range of frequencies. You mentioned putting a 45 degree angle on 2 corners: that is where you could/should fit bass traps which will greatly improve the room's performance below the Schroeder Frequency. Strongly recommended.
A suggestion is to buy a microphone and use the excellent free download REW and measure your room. This will take the guesswork out and provide solid evidence of what any added treatment is doing and also help find the optimum place for any sub/s.
You have what can be considered a small room and as such needs absorption more than dispersion. Absorption must take the form of broad-band construction to be effective. The often recommended: 'just add a rug/curtain/bookcase etc. are only narrow-band absorbers and although they are better than nothing they will not reduce the decay sufficiently across a broad range of frequencies. You mentioned putting a 45 degree angle on 2 corners: that is where you could/should fit bass traps which will greatly improve the room's performance below the Schroeder Frequency. Strongly recommended.
A suggestion is to buy a microphone and use the excellent free download REW and measure your room. This will take the guesswork out and provide solid evidence of what any added treatment is doing and also help find the optimum place for any sub/s.