KDE loves different wallpapers, for some unfathomable reason. Apparently they haven't heard of high-resolution 11520x2160 resolution wallpapers that make my desktop look beautiful ala digitalblasphemy.com since the 90's. Particularly you just need to right click on each desktop and set the background, I tend to do a rolling pattern of 4k displays when using KDE, otherwise I use 4Kx3 display images at full triple 4k resolution that are beautiful when the desktop (like kde) isn't stupid to disallow this.
Cinnamon/mint do this, but do not let you split the screens per desktop. One reason I really hate KDE still. Give me my single framebuffer for 3x 4k displays! Everything else does
-mb