Looks like you have a good plan. I think most will agree that the more bass trapping the better for a small space.
I have a 12 x 12 x 8 space that I had. Here's a blog about it with photos.
http://davidyoungmanmusic.com/blog/m...omplete_studio
I hung four 2' x 4' x 4" bass traps of OC703 and three 2' x 2' x 4" bass traps. Then I have one mobile 2' x 2' bass trap. I also have some skyline diffusors that I made.
According to the calculations I did for absorption I need for this space, I have about half of the recommended amount. After hanging all the acoustic treatment I measured with test tones and an SPL meter. I measured various low frequencies as being a higher volume by about 10db. When I work on projects I notice the 125Hz ringing the most.
The one thing I see from your photo is the panels on the side walls. You'll want to make sure they are positioned for the earliest reflection. Maybe you already know about this. To find the the early reflections sit in your desk chair and have someone move a mirror along the wall. When you see the speaker in the mirror that is the early reflection point. You can also do this on the ceiling, though with your higher celing it may not be as big of an issue.
Your sofa may help with the bass trapping as well.
Have fun building!