In the sand

Also apparently cron works in termux, so it could all probably be scripted there.

I think you could get something working using termux to install and run ffmpeg on your phone. Then using an automation app, e.g. if this then that, you could run a script in termux to re-encode the files. And automate it at night while charging.

Holy cow, didn't realize just how bad Microsoft is getting. That behavior is unjustifiable especially considering this is just for wallpapers.