I used 2 wallpaper applications to achieve this, swww and swaybg

1. For Niri, set layet rule

layer-rule {
    match namespace="^wallpaper$"
    place-within-backdrop true
}

2. Spawn at startup with niri swaybg -i .local/share/backgrounds/paint-blur.jpg &
3. Independent of niri use swww to set wallpaper swww img .local/share/backgrounds/paint.jpg

Hope this helps