Simply replace the Nvidia by the Radeon card and boot a live ISO. Then you know if it’s working within your hardware environment. Choose the open-source video driver if asked. If it’s not working - change it back.
If it’s working remove all specific adjustments from your boot settings (e.g. blacklisting, special kernel modules etc.) If it’s not booting you need to chroot into your system from the live ISO and re-install the right graphic driver within manjaro-chroot environment.