Have you tried to check if the middle carriage is square? I pulled mine apart and when I put a square against the two middle joiner pieces it wasn’t square, it was out by 3 or 4mm. I reprinted one of the middle joiner pieces and when I put it all back together I made sure it was square after every step.
My machine is now perfectly square, after replacing the aluminium tubes with chrome tubes. It’s also much more rigid.