I got this EXACT problem figured out, and I finally got all iterations of URL’s to work listed at the bottom.
Here is the code that works for me.

Proxy:

<VirtualHost *:80>

   ServerName test.x.com
   ServerAlias www.test.x.com
   Redirect / https://test.x.com/

</VirtualHost>

<VirtualHost *:443>

   ServerName test.x.com
   ServerAlias www.test.x.com

   Redirect /www.test.x.com/ /test.x.com/

   RequestHeader set X-Forwarded-Proto "https"
   RequestHeader set X-Forwarded-Port "443"

   <Proxy *>
        Order deny,allow
        Allow from all
   </Proxy>

   ProxyPreserveHost On
   SSLProxyEngine on
   ProxyPass / http://192.168.1.19:80/
   ProxyPassReverse / http://192.168.1.19:80/   

   SSLEngine on
   Include /etc/letsencrypt/options-ssl-apache.conf
   SSLCertificateFile /etc/letsencrypt/live/x.com-0002/fullchain.pem
   SSLCertificateKeyFile /etc/letsencrypt/live/x.com-0002/privkey.pem

</VirtualHost>

This is the end server:

<VirtualHost *:80>
DocumentRoot /var/www/html
</VirtualHost>

In the wp-config.php file, you have to add:

if (!empty($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] === 'https') {
	$_SERVER['HTTPS'] = 'on';
}

That’s it. That’s the magic formula that finally made everything work:

http://www.x.test.com
http://www.x.test.com/wp-admin
http://x.test.com
http://x.test.com/wp-admin
https://www.x.test.com
https://www.x.test.com/wp-admin
https://x.test.com
https://x.test.com/wp-admin

This is a setup for 4 domains on 4 raspberry Pi’s, one of them performing proxy, the other 3 on their own proxied boxes.

I hope this helps someone. I found Apache very simple, but so many out there had code that broke when trying to go to /wp-admin or enter www. The last puzzle was to fix the fact that my cert didn’t support www subdomain, therefore the redirect in the :443 section.

The live site that uses this is test.qso.com and you can see how this code works just as described.