BrewBama asks a key question, are you draining from the low point, or is the bottom surface of the cooler higher at the middle? Can't fix gravity.
Also, your illustration (if it's accurate and to scale) gave me an idea. I recall that Palmer, in How to Brew, has an appendix on lauter tun design and flow and such. He notes that the most even flow in a round cooler (stick with me) is achieved when the braid loop evenly divides the area of the bottom, half inside the braid and half outside. If you square that circle, your illustration of the area you're successfully draining looks a lot like what he describes. So maybe you aren't effectively drawing wort from the ends, because they lie outside the area of idealized flow. This could also mean you are oversparging the middle part of the grain bed and undersparging the ends, which could lead to both unrecovered extract and extracting some tannins and silicates from the oversparged part. If you are experiencing lower than expected efficiency or any other wort quality issues, it could point in this direction.
Maybe redesigning the braid to evenly divide the area of the tun could help.
Or maybe just using the BrewBama system of a "false bottom" by way of rack and grain bag is the simplest fix. Seems like he's got this pretty much dialed in.