Just choose to use the open-source drivers when you install Manjaro and it will work.
iMac 2013 is basically the same hardware as I’m writing this on (Macbook Pro 11,3 Retina 15" Late-2013).
It’s not worth going through the hassle of dealing with ancient unsupported proprietary Nvidia drivers for GT750M graphics that are only 15-20% faster than Intel Iris Pro 5200 anyway. First thing I did after installation was switch to the integrated gpu - less heat and fan noise and better battery life.
Note: on this Macbook Pro 11,3 the bios powers off the igpu unless it detects macOS, not sure if the iMac does the same but if so and you want to use the igpu then you’ll need to chainload apple_set_os.efi or use some other method to persuade it that it’s running in Apple’s walled garden. See this thread for more details.