The next step is mainly done. In the near past I installed another template, but I noticed that Warp7 is nothing for and a little bit to heavy for a fast developed template.
Now I decided to use template and the mobile view is working again. The template does a nice job with Kunena, too.
If you have any issues with the site, don't hestitate and write it to the support forum.
As next steps
- Get all extensions ready for Joomla 3.8 and the up to date version of other extensions
- Migrate the rest of my sites
- Get SchuWeb Sitemap to a stable version