For the most part, there's no real development into the desktop ARM space because A. consumer desktops are dying outside of hobbyists B. ARM research since it's advent has been focused on low-power machines, usually mobile machines. Perhaps when the concept of non-hobbyist desktop PC boards die, there can be a space. Your best bet right now for ARM computer with most things setup is with Pine64. They provide fairly good sbcs, compared to the fruit shit, and although they mostly focus on mobile devices, you could try your hand at running your ZFS server on it. AFAIK FreeBSD doesn't run on them right now.

i was thinking about picking up the MACCHIATObin Double Shot. It has 10 gig network and 3 sata ports. Other then lacking sata ports it seems like it'll do the job. is it a dumb idea to add a USB HDD in a zpool with SATA drives?

ARM boards are shit because of nonstandard boot firmware (openfirmware/u-boot/EFI/BIOS tier bootstrap firmware). A good ARM system is the beaglebone black, since it can be booted without the video decoder firmware, and it is widely supported due to industrial use. The raspi is also popular but frankly you already know both are flawed. For a ZFS server, find a device that is explicitly compatible with a standard FreeBSD or FreeNAS image. It may be better to buy a new or used atom board, since they follow the EFI standard, and some have ECC support, which adds another layer of protection to your filesystem.