Linux

$ fc-match "default font"
NotoSans-Regular.ttf: "Noto Sans" "Regular"

This seems to be correct.

$ fc-match :lang=ja
NotoSansCJK-Regular.ttc: "Noto Sans CJK JP" "Regular"

Also seems to be correct.

I skimmed through the primer and checked whats on the default fontconfig config:

$ cat ~/.config/fontconfig/fonts.conf 
<?xml version='1.0'?>
<!DOCTYPE fontconfig SYSTEM 'urn:fontconfig:fonts.dtd'>
<fontconfig>
 <!-- 
 Artificial oblique for fonts without an italic or oblique version
 -->
 <match target="font">
  <!-- check to see if the font is roman -->
  <test name="slant">
   <const>roman</const>
  </test>
  <!-- check to see if the pattern requested non-roman -->
  <test compare="not_eq" name="slant" target="pattern">
   <const>roman</const>
  </test>
  <!-- multiply the matrix to slant the font -->
  <edit mode="assign" name="matrix">
   <times>
    <name>matrix</name>
    <matrix>
     <double>1</double>
     <double>0.2</double>
     <double>0</double>
     <double>1</double>
    </matrix>
   </times>
  </edit>
  <!-- pretend the font is oblique now -->
  <edit mode="assign" name="slant">
   <const>oblique</const>
  </edit>
  <!-- and disable embedded bitmaps for artificial oblique -->
  <edit mode="assign" name="embeddedbitmap">
   <bool>false</bool>
  </edit>
 </match>
 <!--
 Synthetic emboldening for fonts that do not have bold face available
 -->
 <match target="font">
  <!-- check to see if the weight in the font is less than medium which possibly need emboldening -->
  <test compare="less_eq" name="weight">
   <const>medium</const>
  </test>
  <!-- check to see if the pattern requests bold -->
  <test compare="more_eq" name="weight" target="pattern">
   <const>bold</const>
  </test>
  <!--
                  set the embolden flag
                  needed for applications using cairo, e.g. gucharmap, gedit, ...
                -->
  <edit mode="assign" name="embolden">
   <bool>true</bool>
  </edit>
  <!--
                 set weight to bold
                 needed for applications using Xft directly, e.g. Firefox, ...
                -->
  <edit mode="assign" name="weight">
   <const>bold</const>
  </edit>
 </match>
 <match target="font">
  <edit mode="assign" name="hinting">
   <bool>true</bool>
  </edit>
 </match>
 <match target="font">
  <edit mode="assign" name="hintstyle">
   <const>hintslight</const>
  </edit>
 </match>
 <match target="font">
  <edit mode="assign" name="rgba">
   <const>rgb</const>
  </edit>
 </match>
 <dir>~/.local/share/fonts</dir>
 <match target="font">
  <edit mode="assign" name="antialias">
   <bool>true</bool>
  </edit>
 </match>
</fontconfig>

I tried removing "Synthetic emboldening" here but it doesn't seem to change anything so I put it back. I also tried removing fonts.conf but it still doesn't change anything. My gut feeling is that there is a fontconfig config somewhere changing the way Noto Sans CJK is being rendered in KDE/QT. I just couldn't figure out where or what. The fonts themselves are fine in LibreOffice so I don't think there's any issue with the package.

Now reading through the primer again, I checked the configs in /etc/fonts/conf.d and found all the configs there. There's a lot so I'll look through it and see which one might be changing the way CJK is rendered.

I put it up like this (including just keeping 日本語) and the fonts are still thick :(

That screenshot was just an example but regardless of whether I set my main font as JP/KR/HK/SC/TC etc, the font appears thicker in its "Regular" style unlike non-CJK Noto Sans.

Still haven't found a way to fix it but setting Noto Sans CJK Light as my main font is good enough (see the last picture of my post).