I have an app with a bottom navigation bar that contains the 5 main features. Now I am increasing the number of features to 7. Is it okay to keep the 5 main features visible in the bottom bar, and to add a hamburger menu for the less important features and settings, legal info, user profile, ...? Or should I move everything into the hamburger menu?
By hiding the main features in the hamburger menu, you're making those features less visible.
To quote Don Norman notes in The Design of Everyday Things:
In each state of the system, the user must readily see and be able to do the allowable actions. The visibility acts as a suggestion reminding the user of the possibilities.
Instead, have a hamburger icon in your bottom navigation and show the less regularly used icons in there.
Facebook uses this exact method to ensure that all main navigation is accessible from a single menu (albeit with a modal to dig deeper):