Looks like the current solution is going to be something like:
-Backup EVERYTHING from Storage Barn (my bigass hoarding drive)
-Reformat that to be ZFS
-Move everything back into place
-Backup my Linux root into Storage Barn
-Install NetBSD onto new SSD

I was hoping to try and do something more in-place-y for converting that drive? But when I looked up docs it appears that ZFS wants the entire disk to itself for its pool, so my original plan of "shrink BTRFS volume -> make ZFS volume -> move everything to the ZFS volume -> remove BTRFS volume to reclaim space" wasn't going to work (or if it could have worked, THERE'S NO DOCS FOR IT)