Guessing here, but if you take a look at this StackExchange answer, we see that EFI\boot\bootx64.efi is the fallback bootloader path, and it looks like since it already exists, instead of either failing successfully or prompting to overwrite / rename / etc, it causes a failure state of the bootloader installation script?
However, if you take a look at that page, the fallback is completely optional, and mostly used for booting OSes on removable devices, looks like Windows (of course) just installs a copy there, and grub will as well if it does not already exist, which seems to be the hangup here.
Anyways, I’ll try installEFIFallback to False and see if that works, but may I suggest an option to have a setting for “I already have a bootloader solution” with a helpful reminder of how to run sudo update-grub if selected?