It has gotten a little better by tightening up the belts significantly. The biggest issue I think I have is in the y axis (short rails in my setup). The rail that travels with the middle assembly is not parallel to the other 2 (out by 1/4"). I have checked and the ends are parallel to each other, and my assembly is square. The only thing I can think of is the rollers are not sitting on the rails correctly for some reason, or the middle assembly is not quite square. I checked as best I can with a carpenters square and it looks OK, but it would not need to be off much for 1/4" difference over 24".
I have loosened all of the long bolts that I think would help to the point where I can turn them by hand, rollers and middle assembly. I even loosened the bolts in the middle assembly that hold the two halves together. I also tried to “jump” some teeth in the belts to square it up, but the rail just springs back.
I did a test plot of some diagonal lines, curves, a circle and a square (did not take video) and the plot came out looking good. My square (about 2"x2") was square as well. I believe for my initial usage it will work fine.
I will keep tweaking and trying things on it, but I will be out of town 3 of the next 4 weeks. When I get back next weekend I will do some more plotting and take some close up video.