Can you post this url
cat /etc/default/grub | eos-sendlog
So you say it boots on the live ISO now right to the desktop with amdgpu.dc=0 added on the live ISO? And you said you added it to the installed system in /etc/default/grub command line and then you updated grub with sudo grub-mkconfig -o /boot/grub/grub.cfg but it fails to boot on the installed system?
Edit: If you can boot on the live ISO now is your intent to try a re-install? I have no idea what you have done to it and why it’s not working.