Hi @nico.vadoy!
First for the JSON, you are having some syntax issues. You can use https://jsonlint.com/ to test it.
About updating GeoJSON data, I’ve just released an update for the API to support this. On this page you can see some snippets how to do this automatically:
https://docs.maphub.net/api/map_update.html
I recommend using the Python version, if you haven’t used Python before I totally recommend you to pick it up, from what you’ve done with your map I think Python will be an easy feat for you!