Apologies, I worded that badly, you can just take the route @Volker described, it’s easier 
What I meant in a bit more detail:
When you download & extract the factory images, there’s a flash_fp4_factory.command file in that directory. At the beginning of that file are a few options you can change, one of those is
REBOOT_TO_BOOTLOADER="false" # default value is "false"
If you change that to "true" the phone will reboot to the bootloader instead of Android userland when the installation is complete, which is what we want (and should be default).
But the installation script also compares all the extracted files against the checksums in the SHA256SUMS file to make sure none of the partitions that get flashed is wrong.
Since we modified the flash_fp4_factory.command file the checksums don’t match anymore and the installation fails.
So you have to run sha256sum flash_fp4_factory.command to generate a new checksum, then search for flash_fp4_factory.command in the SHA256SUMS file, and finally replace that line with the new checksum.
Way more complicated, buuut you don’t have to watch the phone to catch the reboot in time 