Thanks for the swift reply. I tried switching partitions from the working mender-converted image setting
fw_setenv mender_boot_part 1
fw_setenv mender_boot_part_hex 1
As suggested here. Is this the correct way to switch partitions in this case?
Funny thing is that after a reboot the device crashed entirely complaining about missing environment variables pxeuuid and bootfile): https://paste2.org/YLGD2k1G 
strange…