fontconfg: Some fonts are too thin to read especially on external monitor

According to you sceeenshot, your anti aliasing is not correctly setup. Depending on your DE, you could activate the anti aliasing and set "Hinting" to full.

On newer Debian/Ubuntu systems the folder you might tweak is /etc/fonts/conf.d. It contains only links to some of the available configurations. Make sure that links to some hinting.conf is contained there:

rwxrwxrwx 1 root root    31 Apr  6 12:09 10-antialias.conf -> ../conf.avail/10-antialias.conf
lrwxrwxrwx 1 root root    36 Apr  6 12:09 10-hinting-slight.conf -> ../conf.avail/10-hinting-slight.conf
lrwxrwxrwx 1 root root    40 Apr  6 12:09 10-scale-bitmap-fonts.conf -> ../conf.avail/10-scale-bitmap-fonts.conf
lrwxrwxrwx 1 root root    39 Apr  6 12:09 11-lcdfilter-default.conf -> ../conf.avail/11-lcdfilter-default.conf

The four links (of many) are mostly responsible for smoothing your font.

In case you don't have any of this, I used to have this in my .fonts.conf:

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>
<!--  Enable sub-pixel rendering --> 
  <match target="font">
    <edit name="rgba" mode="assign"><const>rgb</const></edit>
  </match>
  <match target="font">
    <edit name="antialias" mode="assign">
      <bool>true</bool>
    </edit>
  </match>
    <match target="font">
    <edit name="hinting" mode="assign">
      <bool>true</bool>
    </edit>
   </match>
  <match target="font">
    <edit name="hintstyle" mode="assign">
      <const>hintslight</const>
    </edit>
   </match>
    <match target="font">
    <edit name="autohint" mode="assign">
      <bool>true</bool>
    </edit>
    </match>
    <match target="font">
    <edit mode="assign" name="lcdfilter">
      <const>lcddefault</const>
    </edit>
  </match>
</fontconfig>