4 TB drives are now commonly well under $100 (I've seen them as low as $70) and many Micro-ATX and NUC boards have 4+ SATA connectors. Btrfs has CoW, subvolumes, snapshots, and data integrity features.

For the physical device at least, Debian and 2 Btrfs data drives in RAID 1 certainly isn't turnkey but seems quite accessible at this point.