Hi,
Consider doing it this way:
As for Header/Hero Image section, consider using GP Premium’s Header Element.
https://docs.generatepress.com/article/header-element-overview/
If your header has a slide from a plugin, you should still be able to run it within the Header element using its shortcode.
Once you’ve set a proper hero header, we can now begin inserting a secondary menu by creating it with Block Element.
https://docs.generatepress.com/article/block-element-overview/
Block element lets you create a layout which you can hook in on generate_after_header
with priority set to 20 so it goes in after the Header/Hero section.