Hello.. this has been tried but it doesn't work either. The problem is in the 3740 Bios update that I shouldn't have done and now I'm not able to downgrade..
Sometimes it's possible to force a UEFI/BIOS downgrade by forcing the motherboard to fail POST during boot. After this, the motherboard will try UEFI recovery process. In Asus it's call: ASUS CrashFree BIOS.
Asus motherboards will try to recover seeking the UEFI file in an USB2 drive or DVD (if you have an Optical Drive Connect to your motherboard)
Some motherboards requires the extraction of your UEFI/BIOS firmware place in a FAT formatted USB2 Drive and inserted at a specific USB port (usually with a different color like white)
In some cases in order for the UEFI/BIOS recovery to work you need to rename the UEFI/BIOS file to the name the UEFI recovery expects. This can be found in the motherboards manual but in your case I didn't find info about renaming the UEFI/BIOS file.
On Gigabyte boards you rename the file to GIGABYTE.bin
The way I trigger/force the POST fail is by turning on my motherboard without RAM.
So you can try removing your memory modules and after turning on your motherboard look closlly to your USB drive to see if it's flashing. If it's flashing wait a couple of minutes until it stops flashing. If it doesn't stops flashing then wait about five more minute just to be on the safe side before turning off your board. Then connect back again your memory modules and turn your system. Boot straight into UFI/BIOS Settings and select default settings. Then adjust all the necesarry settings that are convenient for proper functioning of macOS.
Another advantage of doing this is having two different UEFI/BIOS firmware for experimenting if your board has DUAL BOOT. Mine has DUAL Boot so I flip the switch to single boot when I want to try a different UEFI but want to keep the old UEFI version as backup.
If you have your motherboards DVD, then try booting from there or copy the file from there to your USB drive.
Hope this can help you somehow, cheers.