So in May I really thought I had solved the problem of routing mails to Ionos via TLS 1.2 with our Win SBS Server 2008. Looking back, I only had taken one of many cliffs.
If someone is facing the same problem and looking for suggestions: At https://www.cc-community.net/threads/stunnel-konfigurieren-f%C3%BCr-ausgehen... I've summarized how it really works (in German language).