Primary Navigation overlapped and hidden by the main large image at top

Hi there.

looks like you're using the Stretch option to make the Elementor sections full width. I would recommend not using that - instead set the Page Builder Container to Full Width, and set you Elementor sections to the default setting:

https://docs.generatepress.com/article/page-builder-container/

Once thats done we can look a the z-index issue to get the nav in front and the other things you require