The BIOS gives the option of IGPU or PCIe based or auto. I have it set to auto. This is may be a dumb question but if I set the BIOS to the IGPU will I still see the boot information if the monitors are connected to the AMD GPU? It is not easy to get to the cables to switch them if I get a black screen while connected to the AMD GPU or I would experiment to find the answer.
In my BIOS version (it is a maximus a little later, but always maximus is) iGPU can be enabled or disabled (iGPU multi monitor, the second menu item) and when it is enabled other options appear, including the one related to memory. The first field (Primary Display) must be set to Auto (attention because when the second field is enabled, it moves to IGFX and is not good.
So you don't even miss the video in the first part of the startup, keeping the cable connected to the DGPU (RX 580).
It is strange that if you enable the iGPU it will not give you the possibility to do the other relative configurations.
I found on youtube a video about the BIOS of the Maximus VIII and the options correspond to the Maximus X, maybe your problem is that you only acted on the first field and not on the iGPU multi monitor field.