Pretty much the exact reverse of backing it up https://news.ycombinator.com/item?id=29541729

    aws s3 cp s3://... - | gpg -d ... | zfs recv ...

When restoring the order of restores matters, you first need to restore the full snapshot, and then the subsequent incremental ones in order.

I meant, how to test them without incurring a large cost.

Ah gotcha, I haven't done full restore of my main dataset.

I've only verified with a smaller test dataset to validate the workflow on s3 deep archive (retrieval is $0.02/GB). I've done full backup/restore with the zfs send/gpg/recv workflow successfully (to a non aws s3 destination), and used s3 for quite a long time for work and personal without issue, so personally I have high confidence in the entire workflow.