Thanks for that, but as I stated in the OP, the filesystem is f2fs. Apparently, f2fs-tools doesn't have a resize tool, by design. So I guess that means I have to backup my /data partition, format it as ext4 (while shrinking the appropriate length), reformat it as f2fs again, and then restore the backup to /data and hope everything still works.
Edit: It's interesting to note that resize script shrinks by a very paranoid value of 1024K even though the cryptfs footer really only requires 16K. But what's 1MB between friends? 😜