File sync and sharing, plus calendars, contacts, chat and collaborative document editing, all running on a server you control. For schools, clinics, public bodies and anyone else who cannot put files in someone else's cloud, Nextcloud is the established answer, and it has grown into a genuine Google Workspace equivalent rather than a Dropbox clone.
This is the entry on the list with the highest real cost, because someone has to run it: a server, updates, backups and a person who notices when sync breaks. Done properly it is excellent and the data is unambiguously yours. Done casually it becomes the thing nobody wants to be responsible for. Be honest about which you are before you start.