Using A URL As A Data Source
The current URL as a data source is very straightforward -- you give it a URL and it loads it.
You can also load a JSON file from a URL to use as a data source but you should know that it has two limitations:
- It needs to be in our specific format.
- There is a limit on the data size (probably tens of thousands of rows).
Have you considered using either Amazon Athena or Google BigQuery to query this JSON file? (both are supported in Redash)
As for the dashboard -- you can create two dashboards when is the aggregated one and one is the specific one, which will use a parameter for the site it shows data for.