How can I convert .epub files to plain text?

I don't know if Calibre is worth installing for your job, but if you have it you could use the powerful ebook converter:

ebook-convert input.epub output.txt

Output format is deducted from output file extension

I imagine there could be some XML tools/scripts (XSLT) that can transform epub in text as epub is basically XHTML in ZIP archive

An alternative is epub2txt by Kevin Boone, available on Github.

epub2html is a simple command-line utility for extracting text from EPUB documents and, optionally, re-flowing it to fit a text display of a particular number of columns. It is written entirely in ANSI-standard C.

Usage example:

epub2txt input.epub > output.txt

MuPDF can convert from epub to html and txt. To install it:

sudo apt install mupdf mupdf-tools

To use it:

mutool convert -o somefilename.txt somefilename.epub 

It assumes txt output from the -o option.

See mutool convert documentation for more information.