After booting (with Secure Boot inactive), do you see the /boot/efi/EFI/ubuntu/shimx64.efi file? If not, then it sounds like curtin is failing to install it -- either not installing the shim-signed package itself or failing to copy shimx64.efi from that package to the ESP.
After booting (with Secure Boot inactive), do you see the /boot/efi/ EFI/ubuntu/ shimx64. efi file? If not, then it sounds like curtin is failing to install it -- either not installing the shim-signed package itself or failing to copy shimx64.efi from that package to the ESP.