My instinct would be to get rid of this granularity and automatically notify every contact method that a user has set up. I might even require more than one at signup.

The problem that most people who’d use this kind of thing have isn’t missing reminders as much as ignoring them.

Think about a broader target audience of e.g. people who set 10 morning alarms scheduled one minute apart. You could nudge them at the set time and then again every X minutes afterward until they mark the task as completed, only allowing one snooze per task.