I think what you'd need to do is add a criteria to that action group that states that the Completed Date field is null.
That way if it is populated in the future, the criteria for those scheduled actions will have changed causing the actions to fall out of the queue.
There are a lot of rules around scheduled actions and you'd be well served to read this lengthy help topic on scheduled actions to understand what you need to do to get them right.
Specifically note these two bullet points contained within:
- For processes that are set to run when a record is created or edited, scheduled actions remain in the queue only as long as the criteria for those actions are still valid. If a record no longer matches the criteria, Salesforce removes the scheduled actions for that record from the queue.
- For processes that are set to run when a record is created, Salesforce never reevaluates the record with that process. Scheduled actions remain in the queue, even if the record no longer meets the associated criteria when the scheduled actions are executed.
Based on these, you'll also need to ensure that your process runs both when created and when updated.