Assigning a category to a custom post type in Wordpress

Question

I am working with custom posts to create a 'document' custom post type and then trying to assign the category during wp_insert_post, have tried using both post_category and tax_input to no success.

I am registering the custom post as below, keeping in the relevant bits below so it does have $labels defined and a few more $args that I have stripped out.

My research has indicated that permissions need to be created in the custom post type before assigning them to a user role. The user is not logged in, which I understand causes some issues, however in this case I have $user_id passed in which provides the ID in the title and to set the author without a problem so I don't think this is where the problem lies.

Any thoughts on where I am going wrong?

Registering the custom post type:

function document_custom_post_type() {
 
register_taxonomy('document-category', 'Documents',array(
    "hierarchical" => true,
    "label" => "Categories",
    "singular_label" => "document",
    'query_var' => true,
    'rewrite' => array( 'documents' => 'slug name of new registered taxonomy',
    'with_front' => false ),
    'public' => true)
    );
          
    $args = array(
        'labels'              => $labels,
        'supports'            => array( 'title', 'author', 'comments' , 'custom-fields', ),
        'taxonomies'          => array( 'document-category', ),
        'hierarchical'        => true,
        'public'              => true,
        'capabilities'        => array(
            'manage_terms'    => 'manage_document',
            'edit_terms'      => 'manage_document',
            'delete_terms'    => 'manage_document',
            'assign_terms'    => 'edit_posts'
        ),
        'capability_type'     => 'post',
        'map_meta_cap'    => true,
    );
     
    // Registering Document Custom Post Type
    register_post_type( 'document', $args );
 
}
add_action( 'init', 'document_custom_post_type', 0 );

The user role in particular has several caps already, but in this case edit_posts and manage_document are both set to true:

    $client_role = add_role('Client', __('Client'), array(
    'read'                       => true,
    'edit_posts'                 => true,

    'edit_documents'             => true,
    'edit_others_documents'      => false,
    'publish_documents'          => true,
    'read_private_documents'     => false,
    'manage_document'            => true,
    'delete_document'           => true,
    'delete_private_documents'   => true,
    'delete_published_documents' => true,
    'delete_others_documents'    => false,
    'edit_private_documents'     => false,
    'edit_published_documents'   => true,
    
    )); 

When the user verifies their email, I create two custom post types with some pre-defined settings so the example below is for category ID 6 and category ID 4. All fields except the category works, I've included both versions I've tried in the commented out rows:

In functions.php:

            $user_post = array(
                'post_title'     => 'Security Questionnaire '.$user_id.'',
                'post_status'    => 'publish',
                'post_author'    => $user_id,
                'post_type'      => 'document',
                'page_template'  => 'page-templates/document.php',
                //'post_category'  => array( 4 ),
                'tax_input' => array( 'document-category' => array(4) ), //ID = 4 is the tag ID for the Security Questionnaires category
            );
            $post_id = wp_insert_post( $user_post );

And then:

            $user_post = array(
                'post_title'     => 'Contract '.$user_id.'',
                'post_status'    => 'publish',
                'post_author'    => $user_id,
                'post_type'      => 'document',
                'page_template'  => 'page-templates/document.php',
                //'post_category'  => array( 6 )
                'tax_input' => array( 'document-category' => array(6) ), //ID = 6 is the tag ID for the Contract category
            );
            $post_id = wp_insert_post( $user_post );

            $verified_user = new WP_User( $user_id );
            $verified_user->set_role( 'client' );

Can anyone see where I am going wrong? It seems like this should be straight forward but have been struggling with it for a while now.