I'm not dead set on 14TB drives, only that they were on offer. I have about 13TB usable from memory in the Synology, so either option works for me. RAID10 on 14TB drives would be ideal, but I don't have that sort of wedge to drop on drives this side of Christmas... unless I got 2 now and 2 at a later date I guess? Anyone know if FreeNAS supports migrating a RAID mirror to RAID10 per chance?
the deals will come and go. i think if you are plannign on raid10, you really should plan to get all your drives at the same time. it will save you a bunch of tears and effort.
freenas zfs gives your much more flexibility in terms of redundancy and you can mix and match. which is great if you dont have the cash to get all the drives right now, but the downside is the load on the drives when the raid is redistributed.
anyway if you are planning on using freenas, it is a great option to go to. though i have not read too much into the raid building process and how that will affect the drives if such large platter drive (14TB) fails and you replace with another then the raid potentially become vulnerable. have a quick read below, it gives a good introduction. I would say large raid volumn ie you want 3+ disk equivalent of storage space, you need to be looking at 6 bay to 8 bay NAS solutions cos the shear amount of data and the risk of disk falling over during rebuild.
https://www.ixsystems.com/blog/zfs-pools-in-freenas/
before NAS graded HDD drive was a thing, I used to run barracudas in my NAS raids. i remember this time that my 4 bay NAS had a drive fell over. I replaced it with another barracuda drive, but at the end of the rebuild, another old Barracuda failed. that prompted me to replace the enitre raid with Ironwolfs. so things like that can and will happen.