If you intend to stay with Sierra or High Sierra for the foreseeable future, then you can run 2 different GPUs. You can get a stock Mac Nvidia GT120 which is dirt cheap, plentiful and needing no external power other than the PCIe slot as your boot screen GPU and your R9xxx as your main GPU card if you prefer not to flash the boot EFI rom on your R9xxx. You can either have 2 monitors hooked up or like me, have 1 monitor with 2 inputs and switch between inputs. Starting with bootrom 144.0.0.0.0, I am getting consistent intermittent blank screen with my RX580 which never did before the firmware upgrade from MP5,1 after completing boot up and I had to switch between GT120 and then back to RX580 to get the log in screen. Sometimes it's fine without switching and the log-in shows up with the RX580, but sometimes it's not. It's really annoying, but it could be something you are experiencing. That is probably why you may have a log-in screen but it's not showing on your monitor. Which is why it's possible Option key is not working because it's already on log-in. You need an EFI boot GPU capable card to really find out what's going on with your Mac Pro.