can onboard graphics be toggled on and off? I couldn't find the setting in bios.
Yes it can be, options you are looking for is called "iGPU", this board is actually using GPU from CPU if it has one. Board doesn't have gpu on its self.
To disable it:
You must go in BIOS under Advanced - Graphics Configuration - iGPU Multi monitor - Disabled
Also you must select Primary Display and set it to "Auto" or PCIE
I personally don't mind to have iGPU enabled.