I'm without 2.5Gbps+ networking and with less than 20TB of data, so went back to a single large drive (with two duplicates). Moving that much data would take days over 1Gbps ethernet (my "parity" is bit-by-bit drive comparison with freefilesync).

Each 20TB drive today is about $250 renewed. Even with 40TB I'd stick with six bare drives with four USB enclosures. I don't see how you'd get 40TB in a NAS, multiply by 3 for the backups, and upgrade to 2.5 or 10Gbps networking anywhere near your budget.