Trisquel with Xmonad

This might not be a good answer, but as a not-good-with-tech-person I think maybe installing Trisquel mini could work. It already comes with less/other things than Trisquel, then 'work your way backwards' and uninstall the things you don't want/need.
Someone else probably has a good answer for your use case.

If you are well versed on terminal you could start with a netinstall and build from there, be careful with using meta packages as those will call lots of dependencies.

I don't know how efficient is to build all from source also your system will have plenty of development libraries only required to build from source x or y package, but I suppose you could use
sudo apt build-dep package-name
apt source package-name
cd package-name-*
dpkg-buildpackage -us -uc

Some packages could be really resource intensive and take plenty of time to build, but yeah it is possible if you are willing to put the time on such task.

Cheers!

In my understanding, the only effect of installing software you won't use is to take some disk space unnecessarily. Unless you are short on disk space, you may just do a normal Trisquel install and, when you log in, select Xmonad rather than MATE.

To compile Xmonad from source, the easiest might be to follow instructions at https://xmonad.org/INSTALL.html. They use the Haskell package manager cabal but according to https://hackage.haskell.org/upload, packages are verified by trustees and "The code and other material you upload and distribute via this site must be under an open source license.". According to https://wiki.parabola.nu/TPPM_Liberation_Project, cabal is the only third party package manager that Parabola has decided to keep, so that seems to be a good sign.

Have you used sessions with a window manager alone and no desktop environment before? Personally, I use dwm in a MATE session because with dwm alone (I compile it myself because customization is only possible by modifying source code), I find it a bit tricky to configure a number of things, like font size.