Basic question: change the mobile menu options.

Yes, you can customize the mobile menu using the dedicated 'fluent_community/mobile_menu' filter hook.

Use the code below,

add_filter('fluent_community/mobile_menu', function($menuItems, $xprofile, $context) {
    // Remove 'spaces' (groups) from mobile menu
    $menuItems = array_filter($menuItems, function($item) {
        return !isset($item['route']['name']) || $item['route']['name'] !== 'spaces';
    });

    // Add your custom page (library)
    $menuItems[] = [
        'route'    => [
            'name' => 'custom_page',
            'params' => [
                'slug' => 'library'  // Your custom page slug
            ]
        ],
        'icon_svg' => '<svg> your svg code </svg>'
    ];

    return array_values($menuItems); // Re-index array
}, 10, 3);

You can also use a permalink instead of a route:

$menuItems[] = [
    'name'      => 'Library',
    'permalink' => 'https://yoursite.com/portal/library',
    'icon_svg'  => '<svg>...</svg>'
];