How to set category correctly for a custom post created by a remote API call?

1. I'm using a plugin - cleverness-to-do-list (https://wordpress.org/plugins/cleverness-to-do-list/#description ).
2. I'm aware that the plugin is closed, but I've updated it at my end and gotten it working with my self-hosted Wordpress edition (version 6).
3. I'm trying to create a setup where a remote server calls a webpage on my server and POSTs some data.
4. That custom PHP webpage is configured to create a new ToDo item (which is placed in the posts table). It does that fine except for setting the category correctly.

Apologies - am new to Wordpress development, so might be missing something obvious here... How can I create a post and set the category correctly?

The relevant portions of the code: `

    define( 'WP_USE_THEMES', false );
    require_once( $_SERVER[ 'DOCUMENT_ROOT' ] . '/wp-load.php' );
    require_once( $_SERVER[ 'DOCUMENT_ROOT' ] . '/wp-admin/includes/post.php' );
    ...
    ...
    $a2zq3_cat_name = 'Category-name'; 
    $a2zq3_cat_id = get_cat_ID($a2zq3_cat_name);
    $a2zq3_category = get_category_by_slug( $a2zq3_cat_name ); // tried the solution proposed at: https://wordpress.stackexchange.com/a/47423/93042
    
    $a2z3_entry_post = array(
    'post_type'        = 'todo',
    'post_title'       = $a2z3_entry_POSTTITLE,
    'post_content'     = $a2z3_entry_POSTCONTENT,
    'post_status'      = 'publish',
    'post_author'      = 1, // the admin is the author
    'comment_status'   = 'closed',
    'ping_status'      = 'closed',
    'post_category' = array( $a2zq3_category-term_id )
);

    // this works
$a2z3_entry_post_id = wp_insert_post( $a2z3_entry_post, true, false );

    // these work as well
add_post_meta( $a2z3_entry_post_id, '_status', 0, true );
add_post_meta( $a2z3_entry_post_id, '_priority', 1, true );
add_post_meta( $a2z3_entry_post_id, '_assign', -1, true );
add_post_meta( $a2z3_entry_post_id, '_deadline', '', true );
add_post_meta( $a2z3_entry_post_id, '_progress', 0, true );

// these did NOT work, a2zq3_cat_id is the category id (number), a2zq3_cat_name is the category name (string). Both are defined correctly.
    wp_set_object_terms( $a2z3_entry_post_id, $a2zq3_cat_id, 'todocategories');

    wp_set_object_terms( $a2z3_entry_post_id, $a2zq3_cat_id, $a2zq3_cat_name);

    wp_set_object_terms( $a2z3_entry_post_id, $a2zq3_cat_id, 0);

    wp_update_term( $a2zq3_cat_id, 'todocategories', array( 'name' = $a2zq3_cat_name ) );

    do_action( 'add_term_relationship', $a2z3_entry_post_id, $a2zq3_cat_id, 0);`