I’ve done it too and you will be surprised by the audio quality difference 
And man, you are certainly limiting that DAC with the Apple TV.
Curious what Amplifier and Speakers you’ve put behind that DAC.
Guess it isn’t something cheap either 
But that’s off-topic.
The setup is very easy! If you can handle a small screwdriver and connect cables, you can do it.
The manual supplied on the website of IQAudio is very detailed.
IQAudio has some nice digital output boards for adding to your setup, so you can use your DAC to the full potential.
It’s almost plug and play, it’s 1-2-3-play.
Also, it may be worthwhile to push to Cambridge Audio to support Roon in their next firmware update.
At the end, if you are fully satisfied with the new setup, sell your Apple TV (if you don’t use it for other purposes, like Plex) and it won’t even cost you a thing.
Welcome at Roon!