One upon a time I might’ve suggested FreeNAS/TrueNAS but now that unraid supports ZFS there’s not a really compelling reason except if you can’t afford the license fee (which is very reasonable $59-$129 for a lifetime license depending on the license you want. If you don’t want ZFS you can run a standard XFS file system that can be accessed by basically anything. Parity expansion and all the good stuff.
Just saw your additional comment. You can run dockers for Plex and more or full VMs.
They also sometimes do Black Friday discounts to buy a basic license and fully upgrade it).
Look at videos by spaceinvaderone for some examples of what it can do. I have two pro licenses and it’s easily the best computer purchase I’ve made in the last two decades.