Thank you for the response Jimmy.
The question I had with this approach is what to do if they open the app again before the timer has completed. But I think you've addressed that here:
"When your app moves back into the foreground, update your your based on the current time: if any of their timers expired, show the UI as completed; if not, start a new loop to manage the countdown UI from the current point with the time left."
Will just have to try this way for now & will keep an eye out for the updates for iOS15.
Thanks