May be it would be better if you could be a bit more specific on what you are actually wanna to do.
First you wanna archive or backup a whole drive with lots of empty space without saving the empty space. That would make sense on Windows-, maybe other, old, currently not used non-FreeBSD drives/partitions.
Now it's to be automated with a script and done to NFS?

What are you actually going to do?
What I only can guess by your other threads is, you have a bunch of old drives containing lots of stuff from old computers, which you wanna clean up.
Cleaning up old stuff consist of one time tasks - you do it once, but not regulary:
- review what's on a drive: system or files
- decide if it shall be kept or being deleted
- if it shall be kept, decide where and the form, which depends on how often you need to have access to it:
- files best being copied into some directory, or tared & packed
- system partitions/drives are either be kept as they are, shrinked & cloned, or saved as images
Depending on the amount you need to clean up, this can be a bit effort. But if you deal with different drives, containing stuff from different computer systems from different eras, you need to review all yourself what's on it, decide yourself what to keep, how and where. You also need to exchange the drives physically anyway, figure out what partition scheme, partitions and filesystems are on those... I don't see how this really can be automated.
What's done regulary, so being automated, so scripts are useful, are backups: Keep at least one copy of the stuff you currently work with on another physical storage.
But it makes no sense to make copies or clones of full FreeBSD system drives/partitions, especially not as routine backups. The system itself doesn't need to be backupped at all. It's throw away, can be reinstalled anytime. All you keep copies of your system are the config files you edited, only. This would be
/etc/rc.config, maybe
/etc/fstab, and maybe other files under
/etc/, maybe
/boot/loader.conf if you did any changes in that one, and maybe
/usr/local/share/X11/xorg.conf.d/, if you placed some additional config files for some special things for your X server there. If you have cronjobs configured you may also keep a copy of
/var/cron/tabs/ ... - anything you did some editing yourself is only worth to be backupped. And those are a couple of files with about some kB - not even remotely worth to copy the whole drive/partition/pool.
You may also do a
pkg prime-list > /path/to/myinstalledpackages.txt to have a list of what you have installed, which can be used to install it all again automatically when you want/need to set up a new, equal system. But anything else by the system ain't worth to be backupped.
What periodically needs to be backupped, so being automated, is your
/home/ and any other directory keeping
your files in. But those are files, and backupped by some way of copying them.
Neither of those are done by cloning or copying whole drives/partitions: wastes too much time and space and is pretty pointless.