I have invested many many years into research and experiments on Entertainment Setups in Home Environments.
Gathering all my Knowledge, I will now try to answer your question, while explaining you, how you could do it and why.
Let's Start with Apple. Apple is limiting iTunes heavily because of their own interests. iTunes Match is one of them.
To Stream Content to your iDevice, you need a Mac, iTunes, an AppleID and Home Sharing activated.
Otherwise it won't work.
If you have all these things you can only do 1 thing : Stream Music. Nothing Less and nothing more. Simple, Stupid Streaming.
Streaming Content from your Library via Airplay to iDevices.
Well.. in theory iTunes could do it, but practically, it is not possible. Simple as that.
And you are therefore totally right, when saying : Without any other Device, your iTunes Library won't even work at all.
And even if it did with another Device/Machine, it wouldn't work either.
Now we talk NAS.
I have to admit, that the Synology DS have some good UIs for those things.
But if you want to do more than just sharing Music, you are pretty much stuck in the mud.
So how to go about it ?
At first, I would not recommend any of the Apple-Solutions. iTunes is a pain if you want to centralize it.
But it is indeed possible and here is my approach on how to do it :
I have a 10 TB Self-Build Macintosh NAS System. But you could use a FreeNAS System as well. Or even a Ubuntu/Fedora Machine.
(If you don't need iTunes)
I have 6x2TB, 1xTB for the System and stuff, 5x2 TB in a ZFS RAID-Z Pool.
I chose Subsonic for Streaming Music to computers, AudioTap (App for iDevices) for streaming Music to iDevices.
Even though AudioTap is not developed any more, you can absolutely use it. You can Edit Playlists, Give ratings, and so on.
I have a very clean and well Tagged Music Collection that is synced with Subsonic automatically and synced with iTunes Manually.
iTunes Sharing allows every Mac to simply connect to the Music, AudioTap is great for all the iDevices.
But you could also use Subsonic Clients as well.
But this is all Audio.
For Movies and TV-Shows I prefer Plex. And I have some Additional Apps/Clients as well.
Plex could also handle Music, but I prefer Subsonic for that.
Plex can be installed on every OS. Windows, Mac, Linux. Whatever you prefer.
Plex can stream to any device, and you can control it from every Device.
Plex automatically updates the Library if you change something. Plex can handle Playlists.
Both of the Solutions are free. Plex is Free and Subsonic is free (except for streaming to Devices). But you could
use Madsonic instead, which allows streaming without extra fees.
If you have time and Money, I would consider a Mac Mini (older model) in combination with attached Storage.
You can then install iTunes, Subsonic, Plex and many other Apps.
You could also install things like "AirServer", which extends you AirPlay needs.
In fact, the "Media Centralization" is very complicated, if you want to have a good Experience in the end.
I've gone through all of it and even now I am not satisfied with the solution. But there is nothing else.
And iTunes is in my view the worst.
Speaking of iPhoto. You could then also put an Instance of iPhoto onto your mac and Have some script for auto-adding pictures to the library.
I use Piwigo for my Photo-Collection, but I am pretty sure there is something similar to "Automatically add to iTunes" for iPhoto.
All the Other Big Data can be stored on the Server too.
So that is maybe not the answer you wanted to hear, but there is no good way that I am aware of when using iTunes as a Media Server.