Once you start accumulating all the fonts you needed over projects, your Inkscape and Gimp become terribly sluggish on booting. Other programs maybe too, I don't know, I don't use that many graphical tools or office programs.

And the font-selector dropdowns become unusable.

So I started manually moving stuff from /usr/share/fonts/ and ./.local/share/fonts/ into per-project archive folders. A lot of work, that a font-manager could easily do much better. Worse: manually moving stuff from /usr/share/fonts/ can and will bring them back "unexpected" on updates or reinstallation - though there are probably hardly ever updates for font packages.

I would love a tool like fnt to manage this for me, but it seems fnt is more for installing and less for "dis- and enabling" them per theme/project etc.