Hi Jake, now I'm trying to install El Capitan on the same laptop, Dell Latitude E7270 Core i7 DW1830. I use the same EFI that was successfully used in installing High Sierra. But when the USB installer is used to install, the thing that appears is: This version of Mac OS is not supported on this platform! Reason: Mac-473D31EABEB93F9B (as shown in the picture).
Then I change the SMBIOS in config.plist by selecting type 12.1 (as shown in the picture) and after that the USB installer can run. But then it stops as seen in the picture.
Please help.