Classic menus, sub items, & Custom Links on mobile

I’m working on a site based on an older theme (Resi). I’ve switched to the block editor where I can but the site still uses a Classic Menu (Appearance→Menus). My understanding is that contemporary sites use a Navigation block.

For wide screens the menu in the header works fine. Custom Links (#) open sub items and the visual behavior reflects the nested relationships. Artwork reveals items and sub-items, such as Paintings, which reveals deeper sub-items.

For small screens, clicking on a Custom Link closes the menu when IMO it should do nothing. Sub-items are not indented below the first level. It’s difficult for mobile visitors to understand how the site is structured and I don’t understand if this is by design or if something is broken.

I’d appreciate any pointers, thanks.

The page I need help with: [log in to see the link]