I am not a fan of rails_admin. But it really depends on what the users are ought to do.
If your users must use the dashboard to write "posts" for example, rails_admin is an overkill. Writing a controller and some views will take 30 minutes and you can customize them as you wish.
If your users are expected to manage a complex e-commerce application, then maybe you could use rails_admin.
Do use devise. Do not create two models. Have a single User model, with an admin boolean flag.
Control access to the Admin Dashboard with a before_action hook in the Base Controller for the Dashboard itself.