My NAS Build. What about Backup?

All,

I am building a NAS - (specifications below)
Now, I wonder how do you all backup the data.
I am aware of the 3-2-1 backup strategy, but for now, i am looking to use physical drives to backup a copy of the data that is stored on the NAS storage drives and plan to keep it offsite. Haven’t decided on a cloud backup service yet.

Do i need to backup the entire NAS - such as the Cache, Parity and Storage Array or just the storage drives? I lack some clarity here so any help is much appreciated.

Parts

CPU: Intel i5 8500T
Motherboard: Asrock Z370 Pro4
RAM: 2 x 16GB Crucial (32GB total)
Heatsink: Thermalright Assassin X 120R SE Plus
HBA : Adaptec ASR-71605
SSD: 1TB WD Black SN750 (Gen3)
HDD:

  • Parity: 20TB WD Ultrastar SAS
  • Array: 1x 18TB WD Ultrastar SAS
    HDD Cache: TBD
    I also have one spare 20TB WD Ultrastar SAS drive

Cables & Rest from spare i had.