Thanks @claydoh and @Justin
Modifying ~/.config/plasma-org.kde.plasma.desktop-appletsrc
did the trick!
A systemd service sounds like an overengineered solution for this and I am not savvy enough to come up with that anyway.
But eventually I decided to go with the simpler solution of just using the Slideshow mode. My script will fetch a wallpaper image every night and place it in a directory.
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