There is a new, small tool available for KDE & Sparky users: Plasma video wallpaper module.

The module lets you use a video file as an animated wallpaper or lock screen.

It can be installed from Sparky repository via Synaptic or apt commands:
sudo apt update
sudo apt install qml-module-org-kde-video-wallpaper

Then go to your desktop settings and change Image option to Video, then pick a video file.
It needs to log out and log in back.
Tested on Debian testing “Buster” / Sparky 5.

Plasma video wallpaper