I have the exact same workflow except I have two images: one for legacy/MBR and another for EFI/GPT – once I read your post I was glad to see I’m not alone haha!

Started: ~2008 because I saw compiz had the virtual desktop cube & wobbly windows animations. Now I’m on Debian.

I’m a bit late here but when installing grub to a USB drive with a GPT/EFI compatible partitioning, you need to run the following command: “grub-install --target=x86_64-efi --efi-directory=/boot/efi --removable” (without the quotes).