After some initial measurements thru the calibration routine I temporaily added some washer underneath in some spots to somehow level the bed. As the washers are loose changing the bed and repeatability is an issue.
I now did a more precise way and had ordered some thin 0.1/0.2/0.3 stainless steel washers from eBay.
Attached a dial indicator to the Z axis and ran a grid screw hole by screw hole which I tracked in Excel. Front left screw hole in the bed was my reference (zero point).
Bed without any washers:
After super gluing washers as needed to the grid:
I am now level within ~ 0.15mm across the bed. Close enough for the time being… Gotta check if I can get 0.05mm washers…