Not sarcastic, I genuinely like this sort of thing. To each their own.

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:

• File-Tree-Walk
• A simple web server
• XML parsing

Steps are being made toward Guile Emacs integration. The work is mostly being done by Robin Templeton, who (last I heard) works at the Spritely Institute. And as I understand, there are other people pushing on the Guile in Emacs front as well, so you may not have to wait long.

Have you considered trying to setup Kakoune bindings in Emacs? For example like this: https://github.com/jmorag/kakoune.el