Seems my SDM 3055 joined the crowd
Thanks to some prompt and excellent assistance from tautech, the unit is now alive again.
I ended up having to do the internal SD-card recovery route, not ideal. However, having gotten it back alive I noticed that the DMM did not recognize my USB drive!
In Windows everything was just fine, and I repartitioned it several times, including a full "plain" partition (FAT32 all times).
edit: after trying to use my brain for a bit, it hit me. The pen drive was using GPT. I followed this[1] guide (using diskpart, GUI option was not available) to turn it back to MBR and now the DMM recognizes the USB drive.
To upgrade the firmware I ended up using the same SD-card I used for recovery, but with a USB card reader. Somehow it wouldn't let me jump straight from the .19 version (from the recovery image) to the latest .22R1 version, had to do a trip via .20 first.
[1]:
https://docs.microsoft.com/en-us/windows-server/storage/disk-management/change-a-gpt-disk-into-an-mbr-disk