I was having some issues with image flatness on the e-180 and I re-read the manual. There is one paragraph that talks about precise collimation, after the standard collimation procedure. In essence it says that one should rotate the focuser with the collimating tool in it and observe how the center of the crosshairs describes a circle - I guess Takahashi knows that their focusers aren't perfect. The next step they say is to center the secondary on the imaginary center of the circle described by the crosshairs. This is actually not too difficult, in essence one has to tweak the secondary until the crosshairs move evenly within the black dot that is the reflection of the collimating eyepiece.
After that I carefully centered the primary.
This seems to have helped quite a bit. I am at a point of having almost even corner stars with the ASI 6200.
For those struggling with collimation it may be worth trying the same procedure as described in the manual. This is a screenshot of the aberration inspector from NINA.
I have a very small tweak of the secondary to further correct the left corners but that should be trivial.