Headers to non-backport? What kernel are you using?
If you're using the 4.18 kernel from backports, then you need to install nvidia-driver from backports. The stock version in the main Deban repos will error out when you try to build it against a newer kernel.
I think that would be rather evident if you look at the end of the output of "apt-get install --reinstall nvidia-kernel-dkms" with the standard Nvidia version and the 4.18 kernel. That's why I asked if you had seen the message about blacklisting the nouveau driver you get with a successful build...never answered, either.
And to the world...please stop running inxi as root. Just stop.