I've looked at quite a few but none seem to be adept at subscribing to a calendar that requires authentication. Do you have firsthand knowledge of any?

Additionally, our lovely new app doesn't support normal auth login such as "https://user: [email protected]" so finding a service that works is needle in haystack territory.

I have also tried syncing to iOS (only platform that supports full sync from CCS+) but Apple has decided that devices with subscription calendars don't sync to iCloud. So any vanilla "conversation" websites can't see the CCS calendar because it never gets synced off the iPad and onto iCloud.