@memomix ,
Like Fernando said, “You should see a Disk Drive named “PATA” or “PATA SS” within the BIOS. Otherwise no NVMe module will be loaded while booting in UEFI mode”. “PATA” or “PATA SS” should be listed in “Boot Option Priorities”. If “Boot Option Priorities” is still blanked out, it is because there are no bootable drives available (no PATA or PATA SS).
If your “Boot Option Priorities” in your BIOS shows PATA or PATA SS and you install Windows 10 successfully, you will then also have “Windows Boot Manager” in “Boot Option Priorities”. “Windows Boot Manager” is a bootable UEFI drive, in this case it’s a successful install of Windows 10. If you have Windows 7 installed on another drive in the same system and you install it in UEFI mode without any other drives attached, it will also show up in the BIOS as “Windows Boot Manager”, and to boot into that Windows 7 OS you would have to select that “Windows Boot Manager” from “Boot Option Priorities” or the Boot menu by pressing F8 at boot up.
I agree with Fernando, it seems that your modded BIOS was not actually flashed. Also, when you finally see PATA in the BIOS, try not disabling CSM, but still disable Secure Boot and Fast Boot.