Oh, also (probably unrelated to the GStreamer issue from the other thread) I just noticed that the renderer is using the GSettings schema. My suggestion to move them inside the extension directory will make the schema inaccessible for the renderer, since it runs outside the GNOME Shell context. Simplest solution will probably be linking the schema to the other place in postFixup instead of moving it:
postFixup = ''
ln -s "$out/share/gsettings-schemas/${name}/glib-2.0/schemas" "$out/share/gnome-shell/extensions/hanabi-extension@jeffshee.github.io/schemas"
'';