Some suggestions for anyone else looking to do the same:
i3 runs a bit cooler than ryzen, still 8 threads. 8tb WD blues (they're SMR at 8 and up). You can find Atx boards with 8 sata ports and dual nvme slots for caching / fast pools.
I'd be really careful about SMRs in a RAID. You can end up with no end of performance issues. It's all the downsides of a single SMR drive multiplied by however many drives are in the pools.
You actually have to be careful. There's disk sizes where the manufacturer will do say 8TB CMR and essentially the same drive with different firmware as a 10TB SMR. They'll also have a 10TB CMR model. You have to pay close attention to the model numbers. It's even more of a crapshoot if you shuck drives. You have to carefully research what externals are known to have CMRs.
Some suggestions for anyone else looking to do the same:
i3 runs a bit cooler than ryzen, still 8 threads. 8tb WD blues (they're SMR at 8 and up). You can find Atx boards with 8 sata ports and dual nvme slots for caching / fast pools.