This is missing one of the finer points of redundancy: With 4 disks, losing two of them could take out a RAID-10 (both halves of a stripe component), but losing two cannot take out a RAID-6.
The fact that btrfs isn't stable after 15+ years for parity setups is, IMO, unreasonable.
The fact that btrfs isn't stable after 15+ years for parity setups is, IMO, unreasonable.