@gcupc @paulsheprow Haven't looked into it, though I realize Google Calendar is a sleeper pick for hardest Google service to quit.

privacytools.io (a common starting point for me on such questions) doesn't have a section on calendars, but a search of their subreddit may be a good place to start https://www.reddit.com/r/privacytoolsIO/search?q=calendar&restrict_sr=on