@Marcophy or, you can try this yourself

download the run-start.sh script into the MagicMirror folder,
make it executable
change the package.json start script to launch run-start.sh
“start”: “./run-start.sh”,

then make sure you have chromium or another browser and add the
env variable as documented in my script readme(shown as web page for repo on github)

then npm start will launch run-start,
which will launch MagicMirror server mode
and once it’s up, it will launch the browser you select