Override the Steam mangohud config on Steam Deck using a user systemd service. Persistent across updates.
mkdir -p ~/.config/systemd/user
cp example-config/mangohud.conf ~/.config/
cp mangohud.service ~/.config/systemd/user/
systemctl --user daemon-reload
systemctl --user enable --now mangohud.service
systemctl --user disable --now mangohud
# optional
rm ~/.config/systemd/user/mangohud.service
~~~