Having trouble setting the category when creating ToDo item via self made API

Trying to create a setup where a remote server calls a webpage and POSTs some data. That custom PHP page is configured to create a new ToDo item. It does it fine EXCEPT for setting 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' );
...
...
$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',
	);

        // 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);

Apologies – am new to WordPress development, so might be missing something obvious here…