Immutable/offline backups. If you backup to local physical media (HDD/tape), physically disconnect/eject it and store it somewhere safe. If you back up to cloud storage (S3, etc), many of them have immutability options. If configured properly nobody (not even you) can delete or modify the backups (within the specified time period).

After creating it with mkpart, are you formatting it with mkfs.btrfs? You need to both steps (create partition and format it). Also, try running partprobe or rebooting after making changes so that the kernel re-detects it.

There is a filesystem type field in the partition table. Formatting the partition won’t change it. Delete the partition and recreate it with the correct filesystem type. In parted you can do that with “mkpartfs”.

Running it in a dedicated VM is usually the more expensive option, particularly with something simple like DNS.