Why me??
But yeh, I have a Synology as well. Lol
When you have the NAS switched on, does Finder automatically see it? With mine, as soon as the NAS is on, it'll just show up in finder with nothing needing to be done. Also remember the NAS does go to sleep now and then, but the program that's accessing it usually wakes it up automatically with a little delay.
I don't have my complete iTunes library on the NAS itself though. I prefer to have this local on my MacBook for when I sync my devices. So I have my apps, and most of my music local on the iTunes library on my MacBook.
I do, however, use my NAS to store anything else.
Once the iTunes server has been enabled on the NAS, you can place photos, movies and music on the unit. Bear in mind an iTunes server does not do the same thing as your iTunes library. A library is the complete iTunes folder that your device syncs with etc. Content on an iTunes server is simply shared to all iTunes clients on the network so that it can be run/played. You don't actually sync directly with it.
So, as I said, my music and apps stay in my local iTunes library on my local machine for easy and quick syncing. I use the iTunes server on my NAS to store some music, all of my movies, my home videos, and my photos. All the bigger stuff basically. As long as these are all in the correct formats etc, they are all shared automatically into my iTunes on my local machine. So I can view them etc through my iTunes shared folder when I'm on my MacBook just like they were local.
So, we now have content on the NAS that's shared to all iTunes clients on the network.
Next, we want to be able to view all this content directly on any devices, such as my iPhone or iPad etc. I didn't want to keep my MacBook on constantly in order to do this. I wanted a direct connection. Hence I use an app called 'airplayer'. Make sure DLNA on the server is switched on, and this app will then find your NAS straight away. You can then stream your movie, photos, or music straight to your device and watch/listen as though it was on the device locally itself. No mac or MacBook needed in between.
So I now have my content on my NAS. iTunes can see it and uses it as a shared device. My iPad and iPhone can stream data from it directly. But let's take this a step further......
I now want to stream this content straight to other devices. I have an Apple TV2, so want to stream movies straight onto my main TV, again without having to use my MacBook at all just to run iTunes.
The great thing about this AirPlayer app, is that it let's you use Apple Air Play to stream content straight to the Apple TV, or AirPort Express, etc. So now I can use my iPhone or iPad to connect to my NAS and stream movies, photos or music, straight to my Apple TV2. This is actually how I watch most of my movies now. Lol. All works perfect, with no jail breaking involved.
If you want your entire iTunes library itself on the NAS, so that devices sync straight to that directly - although I've not tried it - you could probably move your iTunes folder onto the NAS and point iTunes across to that. The sync speed will depend upon your network speed. Bear in mind you must always be connected to the NAS in order to use iTunes though, or use that library. I've used external HDDs to host my iTunes library before and it seemed to work OK. Don't see why it, would be any different with a NAS....