How do you handle bottom navigation on deeper levels?

We are looking into converting our hamburger navigation into a tab bar, we have too many items in the side menu right now and we want a tab bar with 3-4 items and a menu where the user can customize what appears on the bar.

We want to show the bar on our few top-level views like email, tasks, calendar, etc. But some of these views provide navigation to subviews, we don't want to show the tab bar there to avoid confusion and to save screen space, but the only way to handle that I could find was to hide it upon scrolling down and show it when scrolling up.

We think this is a really ugly solution that wouldn't fit our app and our users at all. Is there any common alternative? How would you handle a bottom navigation system that should not be present in every view?