It sounds like you’ve tried the vast majority of things I would suggest. Without BIOS flashback or some sort of built in mechanism for recovery in the case of a failed update there isn’t a whole lot you can do.
BIOS contains the instructions needed for the machine to power up and boot properly and it’s possible that the instructions are now damaged. I would try booting on a minimum configuration and see if it resolves the error, just in the case that the update resulted in instability or incompatibility with the currently installed parts. Try powering on with just 1 stick of RAM, and the CPU. Unplug literally everything else including any hard drives or storage drives.. I would even remove the graphics card if there’s one installed. You should be able to get video out from the integrated graphics on any CPU that would be compatible with this board.
Otherwise, this might be something you’ll need to contact that manufacturer about. You might be able to find a way to flash a new BIOS directly onto the chip, but I have very little experience with those sort of tools. I know they exist for GPUs and various laptops, but have never sought them out for desktop motherboards.