OK, so the issue isn’t in the hardware/firmware. As @MarekBelisko suggested I tried booting directly the image generated by mkosi, and that works fine (after adjusting the boot order in BIOS).
With the mender-convert created image, after adjusting the boot order in BIOS in the same way that I did for the mkosi image, I get a GRUB screen that I am unfamiliar with. It does not seem to offer an easy way to boot anything:
I did add a sync call to dd command as @mirzak suggested, and also fixed the partition issue after running dd with sudo sgdisk -e /dev/sdc.
I am not sure what to do from here.
Any suggestions?