As far as I can tell, Google Calendar is the only calendar support provided by WTF. Via a conversation on WTF's new Mastodon account, there was interest is supporting calendars from NextCloud and Zimbra.

A calendar module that supports standards such as CalDav and iCAL should allow us to support the most number of calendars with a single module. Maybe, there's are submodules (probably not the right word) but specific auth flows for a provider, but the main calendar data should be pulled via those standards.

I'm going to be fleshing this idea to see if we can get it added to WTF. If anyone has thoughts or wants to tackle this themself, please let me know here.