What made you choose FreeNAS over UnRaid?
I haven’t made my final decision yet. I’ve looked at both. FreeNAS looks to be the more polished product, with better data protection.
Instead of using external controllers, could you have used mobo with many sata ports? I was using an asrock c236 wsi with 8 sata ports no issues. Less compatibility issues than using a external controller.
I can also vouch for the Fractal Design 804, I've used all the bays up but slowly moving to 10tb drives. Total storage is around 50TB atm, also have a Dell Perc H310 & 2x TV cards installed.
If you don't mind a bit of diy I think you will be able to fit a 4 bay caddy in the front if you drill a couple of holes to secure the caddy to the case.