I made a fresh start according to DART-6UL’s board integration page. I have set up an initial build environment for SD-card boot (export MEDIA="sdcard"). Then I applied dellgreen’s local.conf settings with the MENDER_STORAGE_TOTAL_SIZE fit for my eMMC, and it worked! I can boot from eMMC and update the system from the Mender backend. I guess there must have been some residual files in my previous build environment that messed up my images.
Thank you very much for your help!