I would go with Guile, because it is built-in to the Guix Package Manager which is a really good general-purpose package manager.
It ticks several of your boxes:
- has a CLI interpreter
- is a general purpose language, Scheme, amd compliant with revisions 5, 6, and 7 of the language standard
- allows writing in a functional style (it is one of the original functional programming languages)
- small disk footprint, but still large enough to be “batteries included”
- decent documentation, especially if you use Emacs
- simple setup: not so much, unless you are using Guix to begin with. The standard distribution ships with lots of pre-built bytecode files, you need an installer script to install everything.
It also has pretty good libraries for system maintenance and reporting:
Well, Ubuntu or any company could certainly do something like that. But then this company would simply be competing with Android with an incompatible app platform built on top of Linux. App developers who have a hard enough time developing their apps for both Android and iOS would not want to write their app for yet another incompatible proprietary platform, even if the underlying OS kernel was Linux.
As others have said, the real advantage to Linux, the real reason to use it, with desktop environments like Gnome or KDE, in spite of their minor flaws, is that the software is owned by all of us. Unlike proprietary software which you are basically renting for a monthly fee, on Linux you actually own your software and your data.