list of all users subscribed to all board

To get what you're looking for in a single hit, you'll probably need to create a custom endpoint to condense the API requests.  You would likely need to loop through the boards (after performing a LiQL query to get all of the board IDs) and then aggregate all of the results into a single JSON response.

That being said, the restriction of needing to query one board at a time is almost certainly by design because the amount of data that a custom endpoint would attempt to return for all subscribers of all boards would almost certainly exceed the maximum character limit of the API response.

So you may be better off doing the individual board queries and looking through the boards in a script or something.

Hope this helps.