I have the Asrockrack x570 board, its pretty good and also bad at the same time
, the extra it costs gets you dual 10Gb Nics, 10Gb in a NAS is lovely, the ability to do 20Gb transfers even better with the right setup, typically 10Gb NICs take an x8 slot, then a gpu would take an x8 and suddenly you have no more expansion on some things like b550, its very easy to fill sata ports in a NAS, once you add drives for performance and redundancy, what's the sensible min 2 data, 2 parity.
I'm not a flag waiver for the board by the way, its BIOS is atrocious, most consumer boards are much better for tinkering, the IPMI though is awesome, I am always on its KVM and checking its sensors on the web, also whilst dicking about with OSes I was doing remote installs whilst sat in the front room, no need for usb sticks etc, just upload iso from web interface to boot over network, I've basically built it and not had to go near it since, except for adding drives etc, managing it from my laptop, great for something shut in a cupboard, downside of the IPMI is 7w power use even when off which considering I initially tested my build on b450 my idle system consumption jumped from ~20w to ~27w (6 ssd, 6fans)
I did buy some cheap non EEC RAM for it but it failed and all the ffafing trying to get it to work trawling through the rubbish BIOS led my to just buying RAM off the QVL, I bought the 3200 Kingston ECC mentioned earlier, its not super fast but machine has been stable, though I have a non pro APU so ECC is pointless, I didn't think it was bad value and I intended to change CPU at some point when I figured out what I needed.
I actually used a 35w 4000 APU in mine but apart from some game testing of the APU its not been plugged into a monitor, will drop in a 5950 tomorrow.
For your NAS build as your are going full size I would consider upgrading the motherboard to one that gives 8/8/8 on pcie lanes, will give you options for gpu, hba, faster LAN etc. but you can do 2.5Gb on USB quite easily and cheaply and that is worth doing on a NAS at the very least, particulalry as you want to use it as shared storage, 1Gb LAN is much slower than the hard drives, with 2.5/5/10Gb transfers will be upto 2.5/5/10x faster, if you wire in 2 ports you can have double that from multiple sources or combine with SMB multichannel, so your NVMe will be able to send/receive fast transfers for cached data at least.
Blimey if you plan to edit video on NAS storage on 1Gb LAN ...... it is chronic, I transfer many Gbs of video to and from mine and even 10Gb feels like it could do with being faster.
Re-the motherboard the 8/8/4 of the prime pro would probably work, I'd pop the GPU in the x4 chipset slot if it'll boot that way, leaving the full fat slots for the good stuff hba/network expansion or more M2.
Not necessarily so. In Unraid, the cache is just that - a cache for files that live on the main storage drives. So if the cache goes down then the docker files remain on the main storage. Unraid will use the cached files for every day use and, usually every 24hours, copy the changes to main storage.
Unraid is a funny one, stuff can stay on cache and not be on storage, plus it is often moved to storage on a schedule, files aren't cached in a a traditional cache way so a mirror is the way to do it, I have only briefly played with it though, so Imay have missed something.
Unraids reliance on USB drive is quite frustrating, I tried creating that USB on loads of my machines none worked, a 10+year old Vaio laptop eventually saving the day, really needs an iso/install option, such a ballache.