How to prevent WP_Query from filtering on ‘s’ but keep ‘s’ for other purposes?

I am trying to use the pre_get_posts hook to replace WordPress’s search function with my own, so I want to unset $query->query_vars[‘s’] so that it does not restrict the results, but I want to keep ‘s’ as a property of $query so that I can use the get_search_query function. How would I go about doing this?

add_action('pre_get_posts', function (WP_Query $query) {

        if (!$query->is_search()) {
            return;
        }
        if (is_admin()) {
            return;
        }
        $q = get_search_query();
        if (!$q) {
            return;
        }
        $search_parameters = array("q" => $q);
        $search_parameters = apply_filters('custom_search_parameters', $search_parameters);
        $search_results = custom_search($search_parameters);
        if ($search_results) {
            
            unset($query->query_vars['s']); // This causes get_search_query() to no longer work, but omitting it restricts the results.
            $query->query_vars['post__in'] = $search_results['post_ids'];
            $query->query_vars['orderby'] = 'post__in';
            $query = apply_filters('custom_search_query', $query);

        }
});