If your pages are being indexed properly, that is all you need. Google uses the sitemap primarily to audit if they can properly crawl your site and not as a source of URLs TO crawl your site.
Sitemaps are only necessary when a site is extremely large or has content behind a paywall or login. Otherwise, Google will largely ignore the sitemap. There is nothing for you to do.
You will see the pages indexed change up and down from time to time while Google audits your site. This is normal. As well, it is often as much as a couple of months behind.