Just a few considerations:

• For a 12 bay NAS I would strongly consider ZFS - which makes ECC more or less a must.

• Mainboard wise the CWK AMD Board is worth a consideration, and so is the Asrock Live Mixer B850 if you want ECC on AMD5

• A popular build option is using a cheap used or "Chinese" host-build controller as SATA ports are hard to get these days.

• I would personally look at using Proxmox and then TrueNAS as an NAS OS and simply passthrough the HBA.

• Another alternative would be using a Zimbra Board and use their expansion options - but that comes with downsides in terms of CPU power and no ECC.

• For Plex it might be favourable to use a CPU with a built-in GPU for transcoding. Intel is slightly better here, but has other downsides, especially if you want ECC

• Get a Geekworm PiKvm, a original PiKVM, a NanoPi oder JetKVM...or something like that. it's worth it.

• If you don't feel like self-building anymore have a look at the Ugreen. They come without the "only approved HDDs" Synology bullshit, allow you to install your own OS and are fairly capable. But sadly they do not support ECC. (And they aren't really cheaper than self building at least not in central Europe.)

• Self building is absolutely possible and we are here to help you.