... and I'm also not sure I have added it in the correct
location.
$ man 8 grubby
The flags you want to pay attention to are --args --remove-args, and --update-kernel=ALL
Or if you prefer to keep editing /etc/default/grub, just make sure you run
$ sudo grub2-mkconfig -o /etc/grub2-efi.cfg
before rebooting.