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.

Now if I build one myself something I’ve never done is it easy to just move my storage over and not lose any data and be able to access it?

No can't just plug the HDDs into the new machine like you would when switching to another synology. You'll have to copy the data to your new machine.

For DIY consider a setup that supports ECC RAM to help prevent corruption. Any good server motherboard should do.

Unraid is pretty easy to get going on. That's probably the direction I would take in your situation.

Also, if you're not doing 3-2-1 backup now might be a good time to consider an off-site backup plan. That 4-bay Synology at a friend's house with a VPN path would be an option for critical data. You could give them some partitioned space on there and on your new NAS to compensate for the power usage. Setup Borg or Restic and it'll be encrypted on the remote NAS, and benefit from incremental and dedupe to minimize bandwidth usage.