Backup the Time Capsule itself
Apple does not provide an automatic function for this.

You can manually Use Airport Utility and its "archive disk" function to make a snapshot copy of the current Time Capsule drive to an external disk connected directly into the Time Capsule's USB port.

This can take several hours. But if your time capsule dies then you could copy these files back to a new Time Capsule or a repaired Time Capsule.

See pondini.org.