@LoupStyle
If you’re going to install FPOS as a clean install and you are running in the same problems then me, do two things:
before you begin with the FP script in bootloader
fastboot erase avb_custom_key
fastboot --disable-verity --disable-verification
Then modify the flash_fp4_factory.command script:
# Toggles
AUTO_REBOOT="true" # Control reboot behavior. Default is "true"
DATA_WIPE="true" # User data wipe. Default is "true"
DRY_RUN="false" # No-flash run. Default value is "false"
FRP_WIPE="true" # frp wipe. Default is "true"
INTEGRITY_CHECK="true" # Checksum run. default value is "true"
REBOOT_TO_BOOTLOADER="false" # default value is "false"
→ set the FRP_WIPE varaible to false
Then the script should run trough and FPOS should boot.
Else, your bootloader is damaged, as @hirnsushi already mentioned.