Hm ... what is the long goal for GNOME Shell themes btw? Because right now it isn't possible to set a font from Gtk themes, but from Shell themes. And I think most people want to have the same font for Gtk and the Shell, so I would say it might be a good idea to ignore the font-family option in Shell themes completely and simplify the GNOME Tweak font settings to
sans-serif <- GNOME Shell, Gtk Widgets, ...
serif <- Default font for web sites
monospace <- Terminal and editor font
Hm ... what is the long goal for GNOME Shell themes btw? Because right now it isn't possible to set a font from Gtk themes, but from Shell themes. And I think most people want to have the same font for Gtk and the Shell, so I would say it might be a good idea to ignore the font-family option in Shell themes completely and simplify the GNOME Tweak font settings to
sans-serif <- GNOME Shell, Gtk Widgets, ...
serif <- Default font for web sites
monospace <- Terminal and editor font