In your Child Theme - in the functions.php put this code (pls read the comments):

function max_title_length( $meta_title ) {
    $max = 65;

    // Convert array elements into a string, or cast input to string
    $title = is_array($meta_title) ? implode(' – ', array_filter($meta_title)) : (string) $meta_title;

    // Check and trim the title if it exceeds the max length
    if (strlen($title) > $max) {
        // Limit the length and find the last space within the allowed limit
        $title = substr($title, 0, $max);
        $last_space_position = strrpos($title, ' ');

        // Trim to the last full word if possible
        if ($last_space_position !== false) {
            $title = substr($title, 0, $last_space_position);
        }

        // Remove any trailing separator and add " …"
        $title = rtrim($title, ' –') . " …";
    }

    // Return an array if the input was an array, otherwise return a string
    return is_array($meta_title) ? explode(' – ', $title) : $title;
}

add_filter('wpforo_seo_topic_title', 'max_title_length');

The function simplifies the trimming and separator handling to deliver clean results efficiently, while keeping the code easy to read and flexible.