@dustbrother:
Welcome to the Win-RAID Forum!
I generally do not recommend to flash a BIOS, which has been designed by its manufacturer for another mainboard model.
Furthermore it is very easy to get the required NVMe module into any AMI UEFI and rather safe to get it properly flashed and working.
So there is no reason to realize your plan to flash the BIOS, which had been modded by the Forum member hancor for another mainboard model.
Good luck!
Dieter (alias Fernando)

@glorifyday:
Thanks for the link to the latest BIOS for your mainboard.
After having checked it, here is the result of my investigation:
1. The original BIOS named IMAGEFB.ROM doesn't contain any NVMe module.
2. It is no problem to get the uncompressed NVMe module named NvmExpressDxe_4.ffs "as is" inserted into the BIOS by using the AMI MMTool v4.50.0.23.
Here is the related picture after having done the insertion, but before having stored the modded BIOS by using another name:


Now to your additional questions:
1. The stored Windows Key is not affected by any BIOS update (no matter whether you flash an original or an NVMe modded BIOS).
2. After having flashed the NVMe modded BIOS you will be able to successfully flash any other BIOS, which matches your mainboard, but you will lose the NVMe support, if the flashed BIOS doesn't contain an NVMe module.

Good luck!

@glorifyday:
The modded BIOS should and will have the exactly same size as the original BIOS.

@dustbrother:
Here is my comment:
1. To be sure to succeed I recommend to repeat the BIOS flashing by using another small sized FAT32 formatted USB 2.0 flash drive.
2. The CSM BIOS settings "Boot from Storage Devices" and "Boot from PCIe/PCI Extension Devices" have to be set to "UEFI Option ROM", before you start with the OS installation by booting off the USB flash drive.
3. The Kingston disk drive should be removed/unplugged before you start with the OS installation.

Good luck!

@dustbrother:
Due to the current internet problems I can neither see your attached screenshot nor download your attached BIOS.
Nevertheless I will try to answer your questions:
1. AFAIK only a FAT32 formatted USB Flash drive is able to be booted in UEFI mode. As far as I know the tool Rufus solves the size problem itself by deviding the image file.
2. As far as I remember, when I used the ASUS USB Flashback myself the last time, the light was blinking all the time until it got completely off. So a "stable light" during the flashing procedure may indicate, that the flashing failed.

@dustbrother:
1. You should update the Rufus tool. The currently latest version is v3.6.
2. To be able to verify the correct BIOS modding I need the original BIOS as well. Please attach it or post the related download link.

@dustbrother:
Thanks for having attached your modded BIOS. Meanwhile our BIOS modding expert Lost-N_BIOS has given you >here< the link to the BIOS file, where he had fixed the capsule signature.
Questions:
Which is the exact model name of your ASUS mainboard? Is it a MAXIMUS V GENE or a MAXIMUS V FORMULA?
If it should be the "FORMULA" variant (what you had written within your very first post), why did you name your modded BIOSes "M5G"?
This was rather confusing for me and obviously for Lost_N_BIOS as well (look >here<).

By the way: Our very experienced Forum member hancor has just started >here< a new thread, where he offers the NVMe modded BIOS 1903 for the ASUS MAXIMUS V FORMULA. Maybe this will help you too.