I'm designing an app intended to have the following navigation structure: I need to have a welcome view with "Sign up" and "Sign in" buttons as most of apps have:

This view does not show any navigation bar, as it seems to be the common thing. If "Sign In" tapped, then the login view will be presented modally:

And if "Sign up" tapped, the welcome view navigates to a form requesting user input to create an account:

Then, once the user logs in and enters the app, I'd want it to have side menus similar to Facebook, YouTube or Spotify:

Being the central panel a UINavigationController. Left side panel I think it usually is an 'UIViewController'...
The point is, I don“t know what the rootViewController of my app should be, and what hierarchy of view controllers should I have. I've thought about a couple of possibilities:
1) Being the rootViewController an UINavigationController, push the welcome view hidding the navigation bar (is that possible?), presenting the "Sign In" view if needed or pushing the "Sign Up" view. Once the user has logged in, pop these views from the rootViewController, that is a UINavigationController, and then push in such navigation controller the custom view controller managing the side menu stuff.
2) Being the rootViewController the side menu custom view controller, and setting as its central panel an UINavigationController. Push there the welcome view and so on, without setting any view controller for the left/right panels, and then when user has logged in, pop those views from the central panel's