@john-at-foxglove.dev
Thank you for confirming. We tested the USB drive creation steps on Windows 10 and were able to upgrade the device. Can you verify if there is any mismatch by recreating the USB drive? (may be use another USB if possible?) Attached screenshots to help in verifying the intermediate steps.

USB Partitions:

Unetbootin (Drive F:/ is selected as location) :

bootloader order, Esc > Boot From File > BOOT > EFI > BOOT > BOOTx64.EFI (Boots the 20.04 OS):

Once the right EFI file is selected, the Ubuntu 20.04 iso will be loaded and the flash script will be triggered.