@ragged by the way: “And it would be very useful to be able to change the calendar after adding it.” This would be a feature of the calendar app, not of the sync adapter.
[DID YOU KNOW] that you can create address-books, calendars and task lists with DAVdroid?
Just use the small menu icon at the top right of your address book / calendar and tasks overview. This will send a request to your server allowing you to easily create a collection of your choice. This works for address books, calendars and tasks (if the server supports it, which is not mandatory). The last two can be combined also.
Please find some more details inside the image below on what field types are used by DAVdroid to create the collection.