The most primitive test I could think of to look into the vertical inaccuracies that one of my Ender 3 V3 SE produces. All four pillars should be perfectly vertical. Also, the measured distance between the pillar tops and the pillar bottoms should be the same.

In my case, both the x and y-axis produce a tilt, making one pillar tilt 1mm inwards, the opposite side 1mm outwards, and the same for the other axis. Now I have to find a solution to fix this..