Now I found a solution that works for all the apps I tried so far:

• add export GDK_DPI_SCALE=0.5 to /usr/lib/win-kex/xstartup
• activate "Settings Manager" -> Appearance -> Fonts -> DPI -> "Custom DPI Setting" (leaving the proposed 192 next to the checkbox)
• restart XFCE desktop session, and remove all the env QT_SCALE_FACTOR=2 prefixes I've previously applied to various QT-app shortcuts to prevent double scaling.

Thank you for your reply @ka@discuss.tchncs.de !

It seems Kali linux's LibreOffice packages already come with svg icon themes installed out of the box:

ls /usr/share/libreoffice/share/config/*svg*
images_colibre_dark_svg.zip
images_colibre_svg.zip

But somehow that setup is broken. I managed to solve my issue with LibreOffice by uninstalling all LibreOffice related packages from the Kali Linux repositories and instead downloading 7.4.7.2 Linux_x86-64_deb.tar.gz from https://www.libreoffice.org/download/download/ and installing all the *.deb packages it contained.

But sadly, even though I've solved this one, the list of applications I'm missing HighDPI scaling on has grown:

• unison-gtk
• bless
• emacs

If you or anybody else has any hints on what I could try to improve my situation with one of those, I'd love to give it a go! :)