Advice on acoustic treatment

That would be difficult to deal with.... Acoustically.  
Is there any chance you could build a wall out a few feet to mimic the other side?

My situation is somewhat similar to yours, J. Chip, in that my right speaker is about 3’ from its side wall and the left speaker is next to nothing thats in the way. My speakers are 7’ apart and I sit 8’ back from the front plane. And, my room is smaller than yours as well.

My head is also practically against the rear wall and yet, it all sounds good, save for the imaging on the right that can be a touch more forward than the rest of the soundstage. It’s never been a problem once I got used to it and it’s rare as only extreme, right image placements tend to come forward (about 1-2’ forward of the speaker plane: its all in the mix).

My speakers aren’t toed in that much as well and the phantom imaging disappears when I do toe them in, but then the highs hit a little too hard.
It’s always something. 😄

Being that close to the rear wall, I find that first wavefront negates any reflections since its in the near field. Part of the Haas effect is that if the reflected sound arrives less than 40ms after the primary sound, you’ll just perceive it as one sound. Our brains are very good at filtering out reflections, especially in a small room like mine. If possible, maybe you can move your speakers a bit further out into the room and have no need for acoustic treatment at all.

I have nothing behind me but have been thinking of maybe, someday, putting up some spruce sound boards like these: http://www.mother-of-tone.com/acoustic_panel.htm behind me as well as behind the speakers.

All the best,
Nonoise

Talk to GIK Acoustics. I use panels behind the speakers and the couch for the same reason.

If you're the least bit handy, this is what's behind the fabric of an awful lot of very expensive panels: https://www.amazon.com/Owens-Corning-703-Fiberglass-Boards/dp/B005V3L834/ref=asc_df_B005V3L834/?tag=...
Nice thing about this, besides being dirt cheap compared to what they charge for the same thing once they cover it with fabric and call it an acoustic panel, is its super easy to cut to any shape with a sheetrock blade. Cheap and easy enough to experiment. So instead of buying some overpriced panel in some stock shape way bigger than you need you can cut whatever size and cover with whatever fabric you want.

Just remember when it comes to fabric the ones closest to speaker grille material are the most effective at letting the panel absorb. Tight weave dense fabric starts to reflect when stretched over a panel.

agree with Erik and consulting an acoustic treatment company to see what their "solution" is

Another supplier of acoustic panels and material for DIY is ATS Acoustics. 

My dedicated room has Primacoustic panels purchased from Sweetwater Sound that I installed on all four walls plus the ceiling using guidance from Jim Smith's book Get Better Sound. I also used his advice for speaker and seat positions. Made all the difference in the world.

Tom