Solved! Bear in mind for the following post that my SMBIOS is iMac17,1 and has 3 monitors all running over Displayport...
I fixed my graphics problems! I'm not sure what combination of solutions worked here (or whether it was just one), but I tried Lilu+NvidiaGraphicsFixup with EmuVariable64, I tried manually patching the AppleGraphicsDisplayPolicy kext, then I tried the AGDPfix.app solution. It was by this point that I could get to the desktop from boot in full resolution on one screen only. Trying combinations of other screens resulted in non-deterministic booting and non-deterministic screen blackouts. Odd.
Obviously, only 1 out of 3 screens working isn't good enough. What managed to make all 3 work however was altering the Windows 8/10 Support option in my BIOS from Other OS to Windows 8/10. This then toggles a flag which makes the CSM Support (Compatibility Support Module) option available. I set this to Disabled.
Viola! Three working monitors at full 2560x1440 resolution, deterministic booting with boot monitor and login monitor not changing, and the boot process itself is now in full resolution too.
I'm going to go back and remove/undo the changes made (other than CSM) to see exactly what minimum viable configuration works here.
Thanks for your help everyone!