jumping in late as I was travelling.
Anyway for fastboot flashing when writable becomes too large, try to define size of chunks:
$ fastboot -S 100M flash writable…….
Difference between boot from sd card and emmc is in:
aboot and boot image (u-boot binary has hardcoded env path).
As ogra said, we ship in gadget snap binary blobs for both options, and uboot flash will pick correct ones.
Vanilla system boot partition is identical, however at first boot this gets "provisioned" and from then onwards u-boot environments have different values between sd card and emmc
So if you want to strip down gadget snap and generate image with ubuntu-image which can be directly flashed with fastboot to emmc, you need to change both, snapcraft.yaml and gadget.yaml to point to correct blobs for each partition.
ogra is right, from your log, you have likely messed up aboot. It does not seem to pick up emmc boot image, and since it would usually default to fastboot, you are probably running there aboot for sd card boot…..