I may have prepared my media incorrectly.
I used:
pv "/mnt//Downloads/Olarila Mojave 10.14.5.raw.bz2" | bzip2 -cd | dd bs=1M of=/dev/
To watch the progress of the bzip2 being read as it unpacked and wrote to the drive.
Possibly my external drive is failing, it reported no errors, but this 250GB 2.5" HDD is one of two HDDs I have that are larger than 4GB.
Anyway, I proceeded to boot it, and got a mount error trying to mount the rootfs.
It apparently got corrupted somehow, but not irreparably. I was able to perform First Aid on the drive using Disk Utility from my MBP.
Now I have a fully working installation, and I did the EFI step the laziest way possible:
I booted the other OS I kept, an Arch Linux install, and copied the EFI folder from the EFI system partition of the install media to the boot drive, and doctored the config.plist to meet my needs, such as setting darkwake=8 for my MSI board, removing the verbose flag, and enabling RadeonDeInit for my video card. I had to do all of these things for the install process anyway, and I had verified the EFI system worked for booting the installer.
Quick note: The DeInit was definitely needed, as without it, the RX 480 glitched too badly to even progress through the installer.
Bonus: macOS recognized my LG 4k panel as a HiDPI display. Neat!
