Have you tried to do this with AMD graphics? It could actually work much easier.
I really am not in position to give advice on this topic. I’m just curious.
The root of the problem is libva2, which Steam isn’t using because current VDPAU to VAAPI translation layers only support libva1, which the older versions of Debian support, so hence SteamOS not moving to a new version of Debian doesn’t have to use libva2… So…
If libva2 support was official, it means AMD and Intel are fine, but Nvidia is screwed.
So it sounds like it’s a Valve issue? They need to implement support for the new libva2 library?
It’s a Nvidia issue for not modernizing their VDPAU translation layer to libva2…
It’s a Valve issue for basing everything off of older Debian releases, so they don’t see the need for it.
It’s a Ubuntu issue for not providing versioned packaging for the package management of the libva package by just updating major versions in place rather than doing metapackages.
So it’s a combination of a distro oversight and a common infuriating issue when it comes to proprietary (shit) software?
This continues to frustrate me.
So the solution is to throw that 2080 Ti in the trash and get an RX 5700 then… 
Nah, but seriously, this is and will always be the problem with proprietary software. Unless you are allowed to alter the software to run on more modern systems you will inevitably get stuck at some point I’m the future. It is holding Windows back and now it is putting the brakes on (some) Linux (distros), too.