It would be nice to have a simple way to block the user base too

Or run the raid 5 or 6 separately, with hardware raid or mdadm

Even for simple mirroring there’s an argument to be made for running it separately from btrfs using mdadm. You do lose the benefit of btrfs being able to automatically pick the valid copy on localised corruption, but the admin tools are easier to use and more proven in a case of full disk failure, and if you run an encrypted block device you need to encrypt half as much stuff.

If you’re frictionless too, physicists will love you

The wikipedia page has a calculator that supports metric more properly