customer-riverside/web/modules/custom/riverside_pt/riverside_pt.module

<?php

use Drupal\Core\Breadcrumb\Breadcrumb;
use Drupal\Core\Link;
use Drupal\Core\Routing\RouteMatchInterface;

function riverside_pt_system_breadcrumb_alter(Breadcrumb &$breadcrumb, RouteMatchInterface $route_match, array $context): void {
  if ($route_match->getRouteName() === 'riverside_pt.booking') {
    $breadcrumb = new Breadcrumb();
    $breadcrumb->addLink(Link::createFromRoute('← Back', 'riverside_pt.schedule'));
    $breadcrumb->addCacheContexts(['route']);
  }
}

function riverside_pt_mail(string $key, array &$message, array $params): void {
  if ($key !== 'booking_request') {
    return;
  }

  $start = new \DateTime($params['start']);
  $end   = new \DateTime($params['end']);

  $message['subject'] = 'Booking request — ' . $start->format('M j, Y g:i A');
  $lines = [
    'Name:  ' . $params['first_name'] . ' ' . $params['last_name'],
    'Phone: ' . $params['phone'],
    'Slot:  ' . $start->format('l, F j, Y') . ', ' . $start->format('g:i A') . '–' . $end->format('g:i A'),
  ];

  if (!empty($params['comments'])) {
    $lines[] = 'Comments: ' . $params['comments'];
  }

  $message['body'][] = implode("\n", $lines);
}