Yes, that is some error you typically correct by modifying the tower angles slightly. If they are not 100% the height differs a bit depending on position. The escher tool would say you how much but requires z heights at different positions. If I hear half day for that calibration I see that 50 euro for a z probe are well spend since I know you sometimes need to recalibrate if you modified something. In your case move x and y tower towards z tower. Maybe 0.1 degree. Tiny changes can make already a big difference since we want to correct only 0,0x mm normally. I mean it already looks quite good for manually calibrated.
There is also the option to use distortion correction to help with first layer to better stick. While this normally also requires a z probe you can set values also manually.
