My "Bootcamp solution" is . . don't use Bootcamp at all unless you really need to use Win for serious gaming/work projects
I have installed Win 7 & Win 8,1 on their own dedicated spinner HDD's.
It helps if you have a flashed HD 5770, 5870, 7950 = R9 280X to begin with. . plus one Sierra or High Sierra installed drive + a Win7 / Win 8,1 ) target drive + a Win 7, 8,1 install CD ONLY present.
Boot using Option Key.. wait for the EFI screen to come up - your Win 7 or Win 8,1 installation CD should eventually show up. Select Win 7 and boot. . eg as Win 7 boots you'll only see a black screen with a flashing cursor. . . be patient. . .it WILL start the installation process eventually.
HD 7950 + 7970 + R9 280X ( & other GPU's ) all have a 2 bios switch. Assuming that you have flashed your HD7xxx GPU to the the right side bios switch position .. Win 7 at least will be very easy to install.
This worked for me from Mavericks all the way through to High Sierra 10.13.6.
Mojave I just do not trust yet. Apple's ramming unrefined OS updates down our throats without refining previous upgrades is just lazy. Snow Leopard went to 10.6.8 and in it's day was reliable & secure.
As it is we have barely had time to get acquainted with High Sierra = few memorable posts relating to Sierra or HS glaring errors
Maybe I'll go to Mojave a year after if goes GM .. should be interesting to see how Mojave is respected 6 months from now. High Sierra seems at least for me . . stable and efficient. . time will tell.