You'll first have to learn that there are no 'Unix drivers' because the system is
'a POSIX compliant operating system built on top of the XNUkernel, with standard Unix facilities available from the command line interface'
You will then need to learn about Aqua and using Quartz to accelerate it via hooks into OpenGL and also Metal.
Once you have that done you have to maintain the driver for every major release of macOS. You have to keep track of every device ID and vendor ID that comes out and enable them in the drivers.
Then there is OpenCL support. Nvidia with all their money can't get this right and have had a lot of crashes and glitches. Run Luxmark and look at the results to see the errata on Nvidia cards.
Then you have to get CUDA working. Possibly Nvidia's driver will work once you enable the card with the GPU driver.
Once that is done you will have to employ about a dozen people for tech support because if you are going to encourage Mac users to install unofficially supported hardware, when things go wrong then you will have to respond to all the emails.
In total there's about 5 APIs and a kernel to learn. Good luck.