I doubt it is possible to do what you are trying, because Kibana is a single web app and the different screens (Dashboard, Settings, Visualize, etc) are client-side routes of the web app. In other words, the link to Dashboard is qualified with a
# in the URL, and a proxy is not going to see that a user is looking at different screens, because clicking between pages just changes the state of the application in the browser.
If you want to set up a portal where certain users can view only a dashboard, you might just try saving a dashboard in Kibana, use the "embed" HTML source to create a new page you could give to your Dashboard users. See https://www.elastic.co/guide/en/kibana/current/dashboard.html#sharing-dashboards
Note that you'd still need to trust your Dashboard users because they could find the src in the iframe of the embedded Dashboard and find their way to the full Kibana. But even if you did find a way to set restrict certain users to just have Dashboard access, you'd still need to trust that they won't do anything destructive, such as delete a Dashboard.