core-build VS middleware

Have you seen their build guide?

For storage options, most people either purchase a NAS (network attached storage) or re-purpose an older computer using either TrueNAS or unraid. If you're looking to just purchase one, the most popular brand is synology, but their models can be a bit pricey.

https://www.truenas.com/ if you just want to use it as network storage.

Synology if you need prebuilt. If you want to build DIY NAS, take a case that can hold as many drives as you want, take a consumer-grade mobo and Intel/AMD CPU (really doesn't matter for NAS), purchase 1 x SSD for OS and as many drives as you need, deploy something like TrueNAS (https://www.truenas.com/) or Starwinds SAN and NAS (https://www.starwindsoftware.com/san-and-nas), configure RAID (for redundancy, preferably RAID-6) and share the storage to your NUC as iSCSI/NFS/SMB. The second option will require some effort to accomplish but will be more flexible and deliver more performance.

There is no such thing as an ideal OS. Some of the products are better in some of the areas, while other software is better in other areas. For example, Proxmox is the virtualization platform that is targeting virtualization needs. It has support for software RAID, but it doesn't mean that this is the primary feature that is constantly developed. Any NAS OS basically doing the same but targeting storage and sharing things over the virtualization or anything else. So, you need to use whatever is better for the particular use case. Use proxmox on the virtualization host and NAS OS as a storage engine. Or run hypervisor and NAS OS as the VM. As per the alternatives to OMV, you can take a look at Starwinds SAN and NAS (https://www.starwindsoftware.com/san-and-nas), TrueNAS (https://www.truenas.com/), or even pure Debian + Cockpit (https://cockpit-project.org/)

The second is storage. If you need any of the storage sharing, deploy NAS OS as the VM in proxmox, like Starwinds SAN and NAS (https://www.starwindsoftware.com/san-and-nas) or OMV (https://www.openmediavault.org/), or TrueNAS (https://www.truenas.com/). As you mentioned, you need to cross-flash the perc into IT mode and pass through the controller into VM, but you need a separate from the controller drive for proxmox to be able to PCI-E passthrough the card into VM. Then, configure software RAID and reshare the storage to the proxmox via NFS/iSCSI (that will improve your skills in storage stack and storage protocols).

You can also go another route with Proxmox and NAS OS as a VM. TrueNAS or Starwinds SAN&NAS can be used. https://www.truenas.com/

You would need some sort of NAS device to act as a file server (you obviously can't just plug a HDD directly into a switch). Some consumer routers have USB ports where you can plug in an external HDD, though they frequently have speed issues with the USB ports. You could buy something from QNAP, Synology, etc. or build your own TrueNAS.

I run plex on my lab, but if I didn't have that, I would probably buy a cheap server and run TrueNAS. https://www.truenas.com/ I personally have a whole vmware network using it for storage, but as just a plex server with a bunch of storage would be a viable alternative for a single host and have the storage plus the raid benefits of not having to worry about disk failures.

Take a look at TrueNAS or Starwinds SAN&NAS as a NAS OS options. https://www.truenas.com/

Windows built-in iSCSI server is slow. If you don't mind replacing it, try using TrueNAS, Starwinds SAN&NAS, or even Linux (Ubuntu Server) VM running the iSCSI target server. Either solution should overperform the Windows alternative.