2017 27" iMac Keyboard Issue
Solution 1:
I don't know why this is the case, but the USB keyboard does work on boot but only if it's in the second USB port on the back of the iMac. It's the second from the center of the mac, so the third from the outside. Every other USB port doesn't work before the Mac has booted up.
Confirmed with other USB devices as well, that port is the only functioning USB port before I boot into macOS.
It might be a hardware issue with this iMac, I don't have any other 2017 iMacs around me to test.
Thanks for the help guys!