Nice and thorough tutorial!
I followed most of it, but got off the train at Midori since I already had chromium-browser installed on my pi.
What I did instead was modify package.json to add a serveronly script thusly:

  "scripts": {
    "start": "electron js/electron.js",
    "serveronly": "node serveronly"
  },

then modified serveronly/index.js to kick off chrome thusly:

const spawn = require('child_process').spawn
...
console.log("Ready to go! Please point your browser to: http://" + bindAddress + ":" + config.port);
        process.env.DISPLAY=':0.0'
        var browser = spawn('chromium-browser',['http://localhost:8080', '--kiosk', '--incognito'])

Without --incognito I would get an annoying popup from Chrome telling me it didn’t shut down cleanly, and would I like to restore my window configuration.
I then added

su - pi -c "cd /home/pi/Documents/MagicMirror && PATH=/usr/local/bin:${PATH} && /usr/local/bin/npm run serveronly" &

to /etc/rc.local (how’s that for lazy?)
Anyway, HTH

Nice and thorough tutorial!
I followed most of it, but got off the train at Midori since I already had chromium-browser installed on my pi.
What I did instead was modify package.json to add a serveronly script thusly:

  "scripts": {
    "start": "electron js/electron.js",
    "serveronly": "node serveronly"
  },

then modified serveronly/index.js to kick off chrome thusly:

const spawn = require('child_process').spawn
...
console.log("Ready to go! Please point your browser to: http://" + bindAddress + ":" + config.port);
        process.env.DISPLAY=':0.0'
        var browser = spawn('chromium-browser',['http://localhost:8080', '--kiosk', '--incognito'])

Without --incognito I would get an annoying popup from Chrome telling me it didn’t shut down cleanly, and would I like to restore my window configuration.
I then added

su - pi -c "cd /home/pi/Documents/MagicMirror && PATH=/usr/local/bin:${PATH} && /usr/local/bin/npm run serveronly" &

to /etc/rc.local (how’s that for lazy?)
Anyway, HTH