Hi there ... you've got a great looking tank!
I don't think there is any "right" answer here, but the thing that stands out to me is that you have a peninsula tank and are have a dead-spot at the far end (i.e opposite your return and overflow).
In my experience, you want to work WITH the natural circulation pattern setup by your return and overflow, and not setup a powerhead that tries to CROSS the flow.
Using the top photo as a guide, you might try re-directing your return nozzles parallel with the surface and directly at the opposite (far) wall ... this will create a moving sheet of water moving right-to-left across the top of your tank. Absent any power heads, when this sheet of water hits the far end of the tank it has to go somewhere ... in this case it will drop down the face of the far end of the tank. When it gets to the bottom, it will have to therefore turn again and run along the sand from left-to-right ... and then UP the right wall where it will enter the overflow and drain to your sump. In this manner you would have a natural circulation pattern where the water sweeps gently across the sand to bring detritus back to the overflow, and not leave any dead spots.
So ... if that makes sense, you can improve the flow by situating power heads that will accelerate or reinforce this pattern, or create a "gyre" effect by opposing it directly. In my mind that means setting up a power head on the far (left) tank face. However, setting up a powerhead at the back wall of the tank and pushing forward ACROSS this natural circulation is not likely going to facilitate circulation of detritus back to the overflow, and could (I think) make the dead spot problem you are facing now.
Given that you have a shady bottom and the tank doesn't have a lot of depth, setting up a powerhead on the far wall to accelerate the flow pattern created by the returns will probably cause too much blowing sand.
Therefore, setting up a powerhead where the MP40 is and using it to create a counter-flow gyre might get you the outcome you are looking for.
SO before adding more powerhead, I might suggest trying to simply re-position those that you already have to try reinforce the natural circulation in your peninsula.
Then again, I may be over-thinking this.
I'm interested to see how you solve this!
