That's a bit of a tricky one, since the libapache2-mod-php7.2 package is only available in the standard Debian repository from Buster (10) onward. The approach of creating a
php7.1 config file would certainly be ideal, but I can't say for sure if that's an exact possibility at the moment.
The interim solution would be to run php7.1-fpm or php7.2-fpm in a container (docker, systemd-nspawn, etc), or install the equivalent Apache modules using a third party repository such as the one provided by Ondřej Surý.
You can certainly use/upgrade to Symbiosis Stretch in it's current form, but it comes with the caveat that as it isn't 'officially' released yet, some bugs are to be expected. If you'd rather wait for the full release (when it's available in the Bytemark panel), we'll certainly post to let everyone know it's ready.
Unfortunately you can only have one Apache PHP module enabled at any given time, but PHP-FPM packages provide that extra flexibility if you perhaps have a few sites which aren't quite ready for PHP7 yet, and a few others which are.
As an example, some of our servers run a PHP 5.6 Apache module in combination with PHP7.0-FPM and PHP7.1-FPM to run a range of sites with different PHP requirements.