Storj API - Checking if node is online

The API is not a good check, you need to check availability from outside of your network, like UptimeRobot or Kuma.
You may also use a script similar too:

it will detect dates when your node missed audits, but’s not a real time monitoring.

no, but the difference - could be. This is behavior of the dashboard, the API will return a last successful contact date and then you may decide how long ago it was. But you likely want to see it updated at least once in a few hours.