Afaik the filesystem needs to be expanded after being flashed to a sd card, since it still uses the original disk size when the image was created. This isn‘t related to dd.
I took the SD car out again (for the next version I hope they put it on a more easily accessible place by the way) and use gnome-disk to extend the partition without a problem. Thank you.
I thought it was going to be like Lakka. There, you copy the img to the USB and then it expands automatically on the first boot.