I think using page header is actually complicating things as it’s inserting as background image.
If you don’t have any page header content to add to the image, I would recommend adding it as a static image using sections since you are already using it below.
Then you wouldn’t need the CSS to switch it out for mobile as well.
I like the page header because it automatically adapts to the screen to make it full page. I find when using standard method you have to set a fixed height,