The bios string “Flash write protect” is not related to the flash of mod bios files.
Its just a feature like you see in other motherboards (Usually we see a lot in OEM like HP, Lenvo, Dell…) to prevent accidental flashes or bios updates/downgrades.
A mod is always a MOD and standard security (In this case, AMI features) on current motherboards always prevent flashing mod bios files.
Your motherboard model supports USB BFB feature, usually its the best method for flashing mods, AMI tools or regular Asus methods will always fails as you already saw.
The /GAN option was more successful in AMI IV core bioses, very few V will work with the AFUWIN that supports the /GAN cmd.
USB BFB can be very picky in some ASUS boards, regarding USB drive model and size, always the smallest size and different brands is what we need to try.
I had an ASUS that only after CLR CMOS and leaving only 1 stick of RAM, the function worked…
Also i had another ASUS that only accepted FAT16…
As already mentioned in previous posts, Flashrom also worked on several AMI/AMD based bioses.
Other than that, direct programming with a CH341A
The tool AMIBCP will not reflect the changes in AMI V bioses as it was on AMI IV and theres no recent versions leaked around the web…
New method user friendly for modern AMI V bioses is the UEFI tool editor:
[TOOL] UEFI Editor - BIOS/UEFI Modding / BIOS Modding Guides and Problems - Win-Raid Forum (level1techs.com)