Process to encrypt dataset?

[IroesStrongarm]

Depending on the dataset size, pool size, amount of free space available, and some other factors, the unencrypted original may still be recoverable for a while.

This is a good point. For me I'm not concerned about that. I'm encrypting it in advance now so later this year when I setup a remote TrueNAS I'll be able to regularly send an already encrypted dataset without the key as a backup.