The one thing I wouldn’t agree with is ffmpeg.

It does not do one thing. It does a thousand things. The way different functionality works is inconsistent. In some cases you need to read the source code to understand how or why something is happening, as it’s not generated in the already expansive documentation. To me, it’s the antithesis of the UNIX philosophy.

That said, it’s a brilliant piece of software.

Maybe one of the Fedora Atomic distros would be up your alley? https://fedoraproject.org/atomic-desktops/

I don’t think NixOS meets the bill. You’d be learning and troubleshooting a whole new language just to setup your system and modify the core configuration.

I think an open-source general device benchmark would be cool. Including CPU / GPU / Battery life metrics. As far as I know, everything that does this is proprietary.