No support from Radxa - another day spent on the device: I noticed the uEnv.txt has no effect on booting whatsoever. Inspecting the booting process from the serial console more properly finally showed why: the file extlinux/extlinux.conf is found and loaded by the bootloader instead. Removing this file solves both problems: failed booting (due to the wrong path to the DTB) and no serial ports working (due to overlays not loaded at all).
After removing extlinux/extlinux.conf from the boot partition, the boot.scr is correctly loaded instead, and the uEnv.txt is finally applied.
Am I the only person using the Radxa Zero with the new and broken images? Is nobody even testing the images?