Since Bricks 1.3.5 you may manipulate the related posts element query vars before the query is performed like so:
add_filter( 'bricks/related_posts/query_vars', function( $query_vars, $settings, $element_id ) {
$query_vars['post_type'] = [ 'post', 'project' ];
return $query_vars;
}, 10, 3 );
The filter callback receives two arguments:
$query_vars
is an associative array used to feed the WP_Query class$settings
is an associative array containing the element settings set in the builder$element_id
is a string containing the element ID