One troubleshooting technique is to simplify down to bare essentials and add pieces back one at a time until you find the issue. Reset bios to factory and perform minimal changes. This can be more challenging if you aren't sure you can trust your boot drive or memory.
If you're able to get...
If you're looking at FreeNAS don't cheap out on CPU/RAM. ZFS is wonderful but fairly resource intensive as filesystems go. Plex plays quite nicely on top of FreeNAS, but more advanced configurations may require some learning. It's a fun hobby. :)
Can confirm that ubuntu 18.04 can read the HFS+ partition that drive would be using. A live boot disc is probably the easiest way to copy files onto an external drive or dump on some cloud storage.