Before diving in, could you clarify whether you want those sub-menus to be limited expandable/collapsible (only one open at a time, not fully toggleable), or just visually grouped with indentation or divider lines?
Also, how should it behave on mobile — should the same hierarchy appear in the side menu, or be simplified as a flat list?
This is quite a tricky layout to handle with CSS. I explored a similar idea earlier here: Group Tabs on Desktop Menu Tab Bar using CSS.
I haven’t re-tested that code recently, so some parts may need adjustment for the current Glide DOM.