I have never build a machine like that, so I guess I can’t help you much, but like another comment said, it seems like a pain to maintain, I usually have trouble with sata cables losing contact, with that setup there are many cables keen to lose contact.

As for ram I wouldn’t worry about it at all, unless you use zfs 4GB should be more than enough, even 2 or less. Ram is expensive now, so you may want to consider using as little as possible unless you already have it laying around. Does truenas use zfs? If so you may want to use other fs like btrfs or test how well zfs works with the ram you have. I’m not sure zfs is worth the trouble. I wouldn’t buy extra ram.

As for CPU I don’t think it matters much, but like I said, I have never tried your setup. But even an ancient sandy bridge should work fine if it’s just a personal has, with HDDs even with encryption. Works fine on my nas.

Also, if you have access to free old computers you can try a ghetto setup where each each computer only handles 4 drives and then you join them together on a master computer either via nbd or nvme other Ethernet (works on sata too). But that seems like an even bigger pain to maintain and increases your power consumption by a lot.