I would say store a default event together with the pattern it repeats in. Then create exceptions that hold additional info or a cancellation when needed. When they finally add an end date in one way or the other, add the end date to the default.

On load/view time, generate events from the list of defaults and amend with the exceptions as needed. The user should not see the default/exceptions system, just events and the pattern.

I’d say working with exceptions in this way is more efficient than storing many copies of the same event.