Try hanging a quilt or sleeping bag on each of the side walls from immediately in front of each speaker to a few feet back as a test. If it helps, then make some absorption panels for those locations. Some absorption above and between the speakers on the back wall may help as well. You are trying to lower the relative volume of early reflections from the walls around the speakers, because early reflections are more confusing to the brain than later ones.
Hanging any sound absorber on a side wall can be very effective when placed at the first reflection point. To find the first reflection point sit in the listening position and have someone move a mirror along a side wall. When you can see the speaker closest to that wall in the mirror you've found the first reflection point.
I’m by no means a pro at this aspect, moreso have only just gotten lucky with space and placement, but this is definitely a step in the right direction.
From what I see in the provided photo, that reflection point is basically going to be on the walls right in front of the speaker pretty much starting at the front edge. It should probably work to tone down those reflections especially since they appear to be reflecting so quickly they may be almost cancelling each other out by the time the sound gets to your listening position - certainly at least killing that great nearfield stereo image. As you move closer to the speakers, like you described, that first reflection stays aimed behind you to the point you start just hearing the speakers as intended.
As for a wall treatment there, start with two terry cloth bath towels, each one hung on the wall right off the front corner of each speaker. A little blu-tack to hold them temporarily in place should be good enough for the test.
Once you’ve got them hung, head back to your listening spot and see if there’s any improvement. If so, that’s where you need to invent some type of permanent solution since I doubt the bathtowels will fly long term. LOL
One of the cheapest and easiest ways I’ve dealt with reflections and echos in my own listening room was to make fabric wall-hangings. Find a picture frame that’s at least the size of the speaker baffles, or make a frame out of wood (cellular pvc also works great because it cuts cleanly and assembles with that blue plumbers glue in minutes). Then stretch the fabric (we’ll call it art
) over the frame and attach on the back with tape or whatever. You’d want to use a soft, absorbent fabric for this (flannel, thick cotton that doesn’t have a sheen to it, that kind of thing) - artwork or art canvas would NOT be a good choice since it’s a taut and painted or coated reflective surface.
To give you an idea what I’m talking about, here’s the one that’s hanging over my sofa.
That’s just a scrap piece of fabric that fits the ‘style’ of my room stretched over a 4’x 4’ back frame made out of a cut/mitred trim board from HD.
It was cheap, looks like art, and creates a big sound absorbing spot on the wall behind my sofa.
It sounds like you’re definitely limited with how much play you’ve got with movement, so the best way to deal with what you have is to work on those reflections. And this solution may be a reasonable compromise to work within the limitations.
Good luck!