I would like to first enable all possible email notifications by Facebook (including all my Groups and Pages) to then subsequently and selectively turn those off that I don't want.
From the Facebook Help Centre:
You can't turn off notifications entirely, but you can adjust what you're notified about and how you're notified.
Keep in mind that if the notification is from an app, you can block the app on Facebook. To block an app, navigate to its about page. At the bottom-left corner, click Block App.
Email notifications are Facebook updates that you receive via email. To adjust your email notifications from Facebook:
- Click down arrow symbol at the top-right corner and select Settings.
- Click Notifications on the left.
- Select Email.
- From here, you can adjust your email notifications.
In your case choose All notifications, except the ones you unsubscribe from option. And later when you wish to not receive some specific email notification just unsubscribe from email.