Another thing I noticed was a little twist in the Frame. It's a little hard to see in the picture but the seat mounting tabs are bent, the right side frame half is lower at the back tab by about 1/8th of an inch.
So I laid the frame down and gently flexed the frame back with a crowbar. Now despite this frame being 1" high strength tubing there is nothing really preventing the frame halves from moving in the back so it took almost no force to bend it back!
You can maybe see the right half of the frame here where my cast hand is pointing the frame is lower and the seat bracket looks squiggly.