Having both the primary and secondary menu be sticky could cause issues, which is why it’s only possible with one.
That being said, the primary menu could easily be the “secondary” menu – they both have the exact same options.
Things like login/logout should be handled by plugins or custom functions: https://generatepress.com/forums/topic/login-logout-my-account/
Individual menu items can be tweaked to look like buttons as well with some CSS: https://generatepress.com/forums/topic/adding-a-button-to-menu-workaround/#post-228678
Hope this helps 🙂