Looking at your source code, any reason you're doing hand-rolled css, layouts, scrollable divs with main content, inline css?
I remember doing this 15 years ago.
Today this is a problem that was solved dozens of times, from Bootstrap, to Zurb and Tailwind - with productivity, accessibility and variety of devices and screen-sizes in mind.
Your website is unpleasant to use and scroll. It feels you're reinventing the wheel just for the sake of it?