ok, so now there's a solution on the horizon for comfy wallpaper picking via thunar (file manager) on sway / wayfire:
1. package https://gitlab.com/carbonOS/gde/gde-background in debian with picture-uri-dark patched out (one line change)
2. replace swaybg with gde-background
3. set XDG_CURRENT_DESKTOP=GNOME
4. ship thunar 4.8.12 when it comes out