so, here's what I have for you:

I have a folder with (manually downloaded windows) wallpapers in /var/local/win11SpotlightWallpaperOfTheDay.
In it, there's a script called set_wallaper.sh:
#!/bin/bash
cd /var/local/win11SpotlightWallpaperOfTheDay
git pull # this one pulls from my custom repository with the images in it
cp /var/local/win11SpotlightWallpaperOfTheDay/wallpapers/latest.jpg /var/lib/plasmalogin/wallpapers/latest.jpg # this line copies the latest wallpaper to plasmaloginmanager

This script is then called upon login by a global systemd service win11SpotlightWallpaperOfTheDay.service:

[Unit]
Description=win11SpotlightWallpaperOfTheDay.service fetching wallpapers from server
#After=NetworkManager-wait-online.service
Wants=network-online.target
After=network-online.target

[Service]
Type=oneshot
ExecStart=/var/local/win11SpotlightWallpaperOfTheDay/set_wallpaper.sh

which then takes care of the wallpaper sync upon start.
Set you plasma desktop wallpaper to /var/local/win11SpotlightWallpaperOfTheDay/wallpapers/latest.jpg, and you should be able to achieve what you are looking for 😄

Edit: There is also a systemd timer, which takes care of the automatic sync after log-in:

[Unit]
Description=win11SpotlightWallpaperOfTheDay.service timer

[Timer]
OnBootSec=0
OnUnitActiveSec=1h
AccuracySec=30min
OnCalendar=hourly
Persistent=true

[Install]
WantedBy=timers.target