Hi,
If your application is pure Rails, I would advise doing the approach you already mentioned, just a DashboardController with an index/show action would suffice. However, I think that the pure MVC architecture is usually not good enough when developing dashboards. The more components you have in the screen, the more resources you will have to handle in your controller which can compromise the controller's readability.
So, I would either look into adding a design pattern such as presenters or view objects. Otherwise, you can use JavaScript in the frontend to access the various endpoints in the form of controller actions to render specific components of the dashboard. The last option is the best IMHO, because you are going to maintain your application as closer to the MVC as possible (no addition of design patterns), however, it is also important to have some JavaScript knowledge to keep your front end sound.
Hope it helps.
/ Marco