Fixing header on mobile

A site I'm working on has a centred navigation that's been giving me issues. It was stacking, so I added code to make it full width. I had to try various code options because most would send the header button off the side, but got it to work.

Unfortunately, the nav is now really messed up on mobile. It displays the desktop nav plus a hamburger menu. I'd like it to only show the hamburger. But I need to be able to keep the full width on desktop without shifting the button location.

Here's the site: https://tangerine-hypersphere-hh79.squarespace.com/?password=bike

CSS I'm using:

.header-layout-branding-center-nav-center .header-title-nav-wrapper .header-nav {
    width: 100vw;
    display: block;
    position: absolute;
    bottom: -2vw;
    left: 50%;
    transform: translateX(-50%);
}

Thanks in advance!

Thanks so much @tuanphan!