Re: Remember Munich?
Outlook is unfortunately the only simple and cheap(-ish) choice there, even without being tied to an Exchange server.
Owncloud or Nextcloud server, Thunderbird for the client, cardbook plugin for the contacts management on Thunderbird. That gives you a calendar and contacts server you can use for all sorts of stuff. Comes with a fairly decent browser-based setup as well.
You could probably have that up and running inside half an hour. And 2 minutes after that have figured out how to work it properly. If someone with my exceptionally limited technical skill (cf other Comentards) can get it working, you should find it a doddle!.
There really is no excuse to keep forcing outlook on people.