Sadly, too often data protection authorities just ban stuff before providing viable alternatives? I do understand there are issues with privacy shield or gpdr.

For all those people suggesting OSS alternatives, it is difficult. Finding talent (yes, the govt jobs do not pay like private) to install, run, maintain suite of office-based is close to impossible. They tried at our school - as the board tried to ban Teams while they chat in WhatsApp or zoom meetings. We tried SoGo (OSS) calendar - it just sucks. Sync does not happen. The solution always is to restart phone or try later.

These may work for individuals. Not for organisations unless huge number of talent moves to OSS type organisations (tutanota or etc).