increase Pagespeed for Joomla Project

For my Joomla based project I want to increase the page speed. The project is primarily aimed at customers from Germany. In use I have jsitemap pro and JCH Optimize Pro, which has already brought a strong improvement. As shop I use eShop from Joomdonation. The template is from joomlage.com. The Google Pagespeed insight is about 68. With GTMetrix I have a Score of B (87%). [login to view URL] All images are already optimized (with JCH optimize or [login to view URL]). At my hoster [login to view URL] I requested a server move to get a small boost (then with SSD and Ubuntu). This should be done tomorrow and from the score I can see what it has done. I am looking for an experienced SEO specialist who knows where to do what. Changes should be documented and in the best case independent from updates of the shop software, the template or the Joomla core. Thanks for fair offers.