Login as non-admin user removes Dashboard submenu. I wish to re-enable it

It’s not the menu you need to adjust – it’s your roles and capabilities.

By default, a WP Core Author has “Dashboard” in their menu. You’re correct, they do not have a submenu under Dashboard, but clicking Dashboard takes them to the same page as “Dashboard > Home” for other users.

You may want to look through whatever plugins you’re using that may have adjusted permissions, and/or use a user role plugin to check what permissions users have. If they can’t see “Dashboard” at all, they need “read” capability added:

// hook to admin init
add_action('admin_init', 'add_author_caps');
function add_author_caps() {
    // get the author role
    $role = get_role('author');
    // add "read" capability
    $role->add_cap('read');
}

You can add this temporarily wherever you like – theme or plugin – and after you’ve logged in and viewed anything in wp-admin, you can remove this code as WP will save the capability.