I went the safer route and brought a second hand Radeon 7950, which is working fine currently, as I didn't want to do any fiddling, just wanted plug and play..
what riddling would I need to do to use a non-Mac EFI card though? If I understand correctly, I’ve already done the fiddling so can just pop one in? Or would I have further fiddling with future os updates?
I'm already running 10.13.6 / bootROM MP51.0089.B00 so presumably I can just chuck, for instance, a Sapphire Radeon RX 580 in there and go.