Create custom post with meta field with AJAX and the WordPress REST API

I haven’t tested your code, but here the $object is an object and not array:

// Here, $object is a WP_Post object.
function slug_update_post_meta_cb( $value, $object, $field_name ) {
    return update_post_meta( $object->ID, $field_name, $value );
}

And you can actually use register_meta() with a custom post type:

register_meta( 'post', 'myfield', [
    'object_subtype' => 'custom_type', // Limit to a post type.
    'type'           => 'string',
    'description'    => 'My Field',
    'single'         => true,
    'show_in_rest'   => true,
] );

Check my answer here for more details.