Advice on OTiS build (NAS, NZB, Blue Iris)

Hi - firstly thanks for the fantastic website…

I currently have been relying on an old Synology DS418 as a NAS and separate PC build to act as gaming rig and server.

I’ve recently been putting together my own short depth server rack (12U) which mainly houses Ubiquiti UDM-Pro and Switches thus far… I have had on pre-order a Synology RS1221+ but it keeps getting delayed and I then came across the alternative Unraid DIY. I am pretty attracted to this idea since the Synology would have insufficient power to meet my needs so I would have needed a separate NUC anyhow for yet more cost.

Main uses being sought:
Unraid NAS (as much storage as possible but 4 16TB drives in an array + 1 2TB SSD for OS/cache should suffice)
Plex server running on Ubuntu container
NZB, Sonarr, Radarr, Lidarr, Home Assistant on Ubuntu containers
Blue Iris + general workstation (via HDMI to monitor) on Windows container

Current hardware I am thinking of:
|Case: X-Case XG430D - 4U 300mm depth|
|PSU: Be Quiet! Straight Power 450w 80plus gold|
|Motherboard: Gigabyte Intel B365M mATX|
|Processor: Intel Core i5 9400|
|Heatsink: Arctic Alpine 12|
|GPU: Zotac 1060 Mini 6GB|
|Memory: 2x Crucial 16GB DDR4 2666 Dual Rank - CT16G4DFD8266|
|SSD Storage: Sabrent Rocket 2TB NVME M2 Gen 3 2280|
|HDD Storage: WD Red Pro 16TB x4|
|NIC: Mellanox ConnectX-2 10Gbe|
|Fans: 2 in case already (120mm+80mm) but have 2 Noctua NF-A8 PWM (80mm) also I thought I would use for the Synology|

I am afraid I will be told I am crazy thinking this can all fit in that case safely etc but some general questions I had:

MB: should I consider ATX rather than mATX - just concerned re space
HDD: I see a lot of mention of SAS adaptors - does this make a big difference versus Sata 3 in reality?
The case description seems to suggest it can take at least 4 3.5" drives and 1 M2 SSD which I think should work the motherboard
HDD - I see a lot of mention of WD Reds on the site - would it be worth considering the much cheaper EXOS 16TB drives (I think Gen 1 is Sata 3)? (I tried Ironwolf Pro in a synology once but they were clicking every few seconds which drives me mad vs a general hum!)
GPU - to use the server with a windows VM as a desktop as well I believe I need a separate GPU to pass-through or could the I-GPU suffice or would that already be allocated to e.g. Plex server transcoding? Advantage of 1060 is this could be used for light gaming still on the workstation I guess
CPU - I want a decent processor given various roles the server will play but should I consider e.g. the i5 9400T vs i5 9400 given lower TDP?
Case - would I be better off getting a smaller 3U case if possible and then e.g. a separate 1U case to hold HDDs and therefore externalising them?

Any other suggestions / comments much appreciated… no offense will be taken!