You know, I put a RX580 in my 5,1. Updated the bootrom to 14. Upgraded El Cap to Mojave which took my HFS+ drive to APFS, then promptly would not let me create a BootCamp partition to install Windows (even though I would point it to another physical drive) telling me that BootCamp was not available. Apple's support site said it was not going to be possible on the 5,1 and that I needed a 7,1 for it.
So I spent some time thinking about it, and returned to El Capitan, and created the BootCamp drive. It is installing Windows 7 now (and no, it will not be online). Maybe then I will return to Mojave.