So, for now, Schleswig is still running Windows, but has replaced Microsoft Office with LibreOffice on "nearly 100 percent" of machines. It's using Thunderbird for email, calendars, and contacts, talking to Open-Xchange on the back end, alongside the Matrix-based Element for chat and Nextcloud for collaboration.
Schleswig-Holstein is still using Windows, taking it step by step. MS Office to LibreOffice, Thunderbird w Open-Xchange for mail, calendar, contacts, Element/Matrix for chat, Nextcloud for collab.