@jimspoon Welcome to the Win-Raid Forum!
Flashing of a modded, but still capsuled BIOS file without using the ASUS own USB Flashback feature may have been the reason for your shutdown issues.
That is why I recommend
a) to reflash the original BIOS,
b) to remove the BIOS capsule,
c) to modify the .ROM file and
d) to flash it according to >this< Guide.
If you want security, that the insertion of the Nvme module has been done correctly by the UEFITool, please attach or give us a link to your modded BIOS and the original BIOS.
Good luck!
Dieter (alias Fernando)