Dear Asus engineers, I use only ASUS mobo's for over 15 years (I'm on ROG X570-F GAMING now) and nowadays I miss a feature that is implemented only in gigabyte motherboards. It would be awesome if ASUS would implement this feature, too.
Currently in multi-GPU systems (CSM-disabled, UEFI, both GPUs are connected to monitors) the GPU in the 1st PCIe x16 slot is selected as default for EFI framebuffer (struct screen_info). So I can't set the GPU in 2nd/3d slot as default, even though you already have a promising "Tools\GPU Post" tab in UEFI setup (see sample image attached).
When this is needed: When you have more than one GPU and you want to specify where the BIOS / OS bootloader should appear.
What it probably takes: All GPUs are already enumerated in UEFI, so basically you would need to add a button to set the 'Default' GPU and then just fill the screen_info EFI framebuffer struct (GOP) with values not from the 1st GPU, but from the Selected GPU.