3.1. Set Up Custom Route

custom-route.php

<?php
register_rest_route( 'rave-initiative/v1', '/initiative/(?P<id>[\d]+)', [
    'methods'             => WP_REST_SERVER::EDITABLE,
    'callback'            => 'update_initiative',
    'permission_callback' => 'check_initiative_permissions',
] );

function check_initiative_permissions( WP_REST_Request $request ) : bool {
    $post_id = $request->get_param( 'id' ) ?? 0;

    if ( ! is_user_logged_in() ) {
        return false;
    }

    $post = get_post( $post_id );

    if ( null === $post ) {
        return false;
    }

    if ( current_user_can( 'edit_published_posts' ) ) {
        return true;
    }

    return get_current_user_id() === $post->post_author;
}

function update_initiative( WP_REST_Request $request ) : WP_REST_Response {
    $post_id      = $request->get_param( 'id' );
    $block_id     = $request->get_param( 'block_id' );
    $post_content = get_post_field( 'post_content', $post_id );
    $post_blocks  = parse_blocks( $post_content );

    $post_blocks = array_map( function( $block ) use ( $block_id ) {
        if ( 'rave/initiative-tracker' !== ( $block['blockName'] ?? '' ) || ( $block['attrs']['id'] ?? 0 ) !== $block_id ) {
            return $block;
        }
        
        // Update block attributes here...
        // E.g., $block['attrs']['some-attr'] = $attr_value;

        return $block;
    }, $post_blocks );

    wp_update_post( [
        'ID'           => $post_id,
        'post_content' => serialize_blocks( $post_blocks ),
    ] );

    return new WP_REST_Response( __( 'Initiative updated.', 'resource-tracker' ), 200 );
}