On our very first attempt to convert the Dell Latitude E6540, we took the ESP contents from the Dell Latitude E5440 and attempted to boot it without changes since doing so was successful on the larger Dell Latitude E5540. It did not take long for us to see that this was a mistake.
After taking some time to review the verbose mode from the initial attempt, we saw that we needed to rebuild the EFI folder from the ground up so we did and then went for a second attempt. This attempt turned out to be 99% perfect due to one of the installed kernel extension which caused an issue with the fresh installation.
If we had installed macOS to the boot storage while it was connected to a desktop Mac in one of those SATA to USB enclosures, we would not have had this issue since the EFI folder supported booting but not installation so that gave us a chance to find a potential bug before release.
Fortunately, finding the kernel extension was going to be an easy task as it is being used on the Dell Latitude E5440 so the kext was copied and then imported for a third attempt. The third attempt was a success and the System Software is installing.