Web template confusion (can't turn off buffering)

As you are using Nginx+Apache2:

Web Template APACHE2
Uses the templates located in /usr/local/hestia/data/templates/web/apache2/php-fpm/

Backend Template PHP-FPM
Uses the templates located in /usr/local/hestia/data/templates/web/php-fpm/

Proxy Template
Uses the templates located in /usr/local/hestia/data/templates/web/nginx/

So take a look to this post, it could be useful for your use case:

Note: In above post, newtemplate.{tpl,stpl} must be located in /usr/local/hestia/data/templates/web/nginx/

That’s because you are not using Nginx standalone so you don’t use fast_cgi cache provided by Nginx.