Therefore, I wrote a small tool, that lists all uninstallable packages sorted by the amount of space that gets freed when uninstalling them. One can navigate through the list interactively and mark which packages should be uninstalled and which should be keep "forever" or "for now". When starting the tool later again, it remembered the packages you want to keep and hides them so that you can continue where you stopped the triage the last time.
I thought it might be handy for others too, so here it is on Github and there's a PKGBUILD in the AUR.
Happy Hacking,
Nudin