Storage & Backup solution – Homeserver with 2 drives

My Debian 12 server just got two brand new Toshiba N300 4TB drives installed. I also have a 1TB external drive connected.

I’m looking for good backup/storage solutions for this setup. As I understand it, the external drive has no place in any sort of array-solution, so I’m aiming to use it for occasional cold backups.

But what to do with the two internal 4TB drives?