I've personally never experienced this issue with the hashrate dropping. Sluggish response yes, but never lower performance. If you do want to activate the onboard graphics, a slightly better option is to buy "hdmi => vga" converter. They cost the same or ever less than those dummy plugs, do the same thing for mining purposes, but have an actual function as well besides just being a useless plug.
Yeah thanks but I was kinda hoping somewhere there's a software option. But I guess not
Have a few of those boards myself, z270p and z270e, couldn't find anything in the BIOS to activate the onboard without a physical plug.