ZFS eats memory for breakfast. All thats stopping a Pi being a really great BSD+ZFS filestore is the USB bound disk access and the lack of memory. The sweetspot would be north of 4GB, but probably 8GB would be enough. SATA would totally rock because you can get SATA enclosures which handle all the hot swap you need, and the bandwidth is fine.

So.. you can get a SATA Pi-Hat. But.. you can't boost memory north of 4GB. (sigh)